From f4d4a59d47400ea729195c9565ef6ecbffeb5c87 Mon Sep 17 00:00:00 2001
From: "okbob@github.com" <okbob@github.com>
Date: Mon, 3 Jan 2022 07:51:07 +0100
Subject: [PATCH v20220126 1/3] session variables

This is new class of database objects. Implemented session variables has
persistent (or temporary) entry in system catalog (like tables, sequences,
...), but the value is stored in session (unshared) memory. The access rights
(read, write) can be granted to others (not owner) users.

The session variables are not transactional like variables in PL/pgSQL or
any session variables in other database systems (package variables, schema
variables, ...).

This patch introduce new DDL commands: CREATE VARIABLE, DROP VARIABLE and DDL command
LET. Commands GRANT, REVOKE, DISCARD, ALTER are enhanced to support session
variables too.
---
 doc/src/sgml/advanced.sgml                    |   59 +
 doc/src/sgml/catalogs.sgml                    |  144 +++
 doc/src/sgml/config.sgml                      |   15 +
 doc/src/sgml/glossary.sgml                    |   16 +
 doc/src/sgml/plpgsql.sgml                     |   13 +
 doc/src/sgml/ref/allfiles.sgml                |    4 +
 .../sgml/ref/alter_default_privileges.sgml    |   14 +-
 doc/src/sgml/ref/alter_variable.sgml          |  179 +++
 doc/src/sgml/ref/comment.sgml                 |    1 +
 doc/src/sgml/ref/create_variable.sgml         |  193 +++
 doc/src/sgml/ref/discard.sgml                 |   14 +-
 doc/src/sgml/ref/drop_variable.sgml           |  120 ++
 doc/src/sgml/ref/grant.sgml                   |   23 +
 doc/src/sgml/ref/let.sgml                     |  107 ++
 doc/src/sgml/ref/pg_restore.sgml              |   11 +
 doc/src/sgml/ref/revoke.sgml                  |    6 +
 doc/src/sgml/ref/set_role.sgml                |    2 +-
 doc/src/sgml/reference.sgml                   |    4 +
 src/backend/access/transam/xact.c             |   11 +
 src/backend/catalog/Makefile                  |    4 +-
 src/backend/catalog/aclchk.c                  |  307 +++++
 src/backend/catalog/dependency.c              |   18 +-
 src/backend/catalog/namespace.c               |  371 ++++++
 src/backend/catalog/objectaddress.c           |  126 +-
 src/backend/catalog/pg_shdepend.c             |    2 +
 src/backend/catalog/pg_variable.c             |  396 ++++++
 src/backend/commands/Makefile                 |    1 +
 src/backend/commands/alter.c                  |    9 +
 src/backend/commands/discard.c                |    6 +
 src/backend/commands/dropcmds.c               |    4 +
 src/backend/commands/event_trigger.c          |    6 +
 src/backend/commands/explain.c                |   16 +
 src/backend/commands/seclabel.c               |    1 +
 src/backend/commands/sessionvariable.c        | 1109 +++++++++++++++++
 src/backend/commands/tablecmds.c              |    1 +
 src/backend/executor/Makefile                 |    1 +
 src/backend/executor/execExpr.c               |   55 +
 src/backend/executor/execExprInterp.c         |   12 +
 src/backend/executor/execMain.c               |   49 +
 src/backend/executor/execParallel.c           |  148 ++-
 src/backend/executor/spi.c                    |    3 +
 src/backend/executor/svariableReceiver.c      |  145 +++
 src/backend/jit/llvm/llvmjit_expr.c           |    6 +
 src/backend/nodes/copyfuncs.c                 |   41 +
 src/backend/nodes/equalfuncs.c                |   36 +
 src/backend/nodes/outfuncs.c                  |   22 +
 src/backend/nodes/readfuncs.c                 |    4 +
 src/backend/optimizer/plan/planner.c          |    8 +
 src/backend/optimizer/plan/setrefs.c          |  113 +-
 src/backend/optimizer/util/clauses.c          |   69 +-
 src/backend/parser/analyze.c                  |  282 ++++-
 src/backend/parser/gram.y                     |  212 +++-
 src/backend/parser/parse_agg.c                |    6 +
 src/backend/parser/parse_expr.c               |  204 ++-
 src/backend/parser/parse_func.c               |    4 +
 src/backend/parser/parse_target.c             |    4 +-
 src/backend/parser/parser.c                   |    3 +-
 src/backend/rewrite/rewriteHandler.c          |   34 +
 src/backend/rewrite/rowsecurity.c             |    8 +-
 src/backend/tcop/dest.c                       |    7 +
 src/backend/tcop/pquery.c                     |    3 +
 src/backend/tcop/utility.c                    |   34 +
 src/backend/utils/adt/acl.c                   |   21 +
 src/backend/utils/adt/ruleutils.c             |    9 +
 src/backend/utils/cache/lsyscache.c           |   13 +
 src/backend/utils/cache/plancache.c           |   34 +-
 src/backend/utils/cache/syscache.c            |   23 +
 src/backend/utils/fmgr/fmgr.c                 |   16 +-
 src/backend/utils/misc/guc.c                  |   11 +
 src/bin/pg_dump/common.c                      |    3 +-
 src/bin/pg_dump/dumputils.c                   |    5 +
 src/bin/pg_dump/pg_backup.h                   |    2 +
 src/bin/pg_dump/pg_backup_archiver.c          |   12 +-
 src/bin/pg_dump/pg_dump.c                     |  237 +++-
 src/bin/pg_dump/pg_dump.h                     |   25 +-
 src/bin/pg_dump/pg_dump_sort.c                |    6 +
 src/bin/pg_dump/pg_restore.c                  |    9 +-
 src/bin/pg_dump/t/002_pg_dump.pl              |   65 +
 src/bin/psql/command.c                        |    3 +
 src/bin/psql/describe.c                       |   94 ++
 src/bin/psql/describe.h                       |    3 +
 src/bin/psql/help.c                           |    3 +-
 src/bin/psql/tab-complete.c                   |   67 +-
 src/include/catalog/dependency.h              |    5 +-
 src/include/catalog/namespace.h               |    6 +
 src/include/catalog/pg_default_acl.h          |    1 +
 src/include/catalog/pg_proc.dat               |    3 +
 src/include/catalog/pg_variable.h             |  117 ++
 src/include/commands/session_variable.h       |   44 +
 src/include/executor/execExpr.h               |   10 +
 src/include/executor/execdesc.h               |    4 +
 src/include/executor/svariableReceiver.h      |   25 +
 src/include/nodes/execnodes.h                 |   19 +
 src/include/nodes/nodes.h                     |    2 +
 src/include/nodes/parsenodes.h                |   43 +-
 src/include/nodes/pathnodes.h                 |    3 +
 src/include/nodes/plannodes.h                 |    4 +-
 src/include/nodes/primnodes.h                 |   19 +-
 src/include/optimizer/planmain.h              |    2 +
 src/include/parser/kwlist.h                   |    3 +
 src/include/parser/parse_expr.h               |    1 +
 src/include/parser/parse_node.h               |    3 +
 src/include/parser/parser.h                   |    6 +-
 src/include/tcop/cmdtaglist.h                 |    5 +
 src/include/tcop/dest.h                       |    3 +-
 src/include/utils/acl.h                       |   10 +-
 src/include/utils/lsyscache.h                 |    1 +
 src/include/utils/syscache.h                  |    6 +-
 src/pl/plpgsql/src/pl_exec.c                  |   55 +
 src/pl/plpgsql/src/pl_funcs.c                 |   24 +
 src/pl/plpgsql/src/pl_gram.y                  |   28 +-
 src/pl/plpgsql/src/pl_reserved_kwlist.h       |    1 +
 src/pl/plpgsql/src/plpgsql.h                  |   14 +-
 src/test/regress/expected/misc_sanity.out     |    4 +-
 src/test/regress/expected/sanity_check.out    |    1 +
 .../regress/expected/session_variables.out    |  849 +++++++++++++
 src/test/regress/parallel_schedule            |    2 +-
 src/test/regress/sql/session_variables.sql    |  614 +++++++++
 118 files changed, 7343 insertions(+), 81 deletions(-)
 create mode 100644 doc/src/sgml/ref/alter_variable.sgml
 create mode 100644 doc/src/sgml/ref/create_variable.sgml
 create mode 100644 doc/src/sgml/ref/drop_variable.sgml
 create mode 100644 doc/src/sgml/ref/let.sgml
 create mode 100644 src/backend/catalog/pg_variable.c
 create mode 100644 src/backend/commands/sessionvariable.c
 create mode 100644 src/backend/executor/svariableReceiver.c
 create mode 100644 src/include/catalog/pg_variable.h
 create mode 100644 src/include/commands/session_variable.h
 create mode 100644 src/include/executor/svariableReceiver.h
 create mode 100644 src/test/regress/expected/session_variables.out
 create mode 100644 src/test/regress/sql/session_variables.sql

diff --git a/doc/src/sgml/advanced.sgml b/doc/src/sgml/advanced.sgml
index 71ae423f63..97470521a8 100644
--- a/doc/src/sgml/advanced.sgml
+++ b/doc/src/sgml/advanced.sgml
@@ -700,6 +700,65 @@ SELECT name, elevation
   </sect1>
 
 
+  <sect1 id="tutorial-session-variables">
+   <title>Session Variables</title>
+
+   <indexterm zone="tutorial-session-variables">
+    <primary>Session variables</primary>
+   </indexterm>
+
+   <indexterm>
+    <primary>session variable</primary>
+    <secondary>introduction</secondary>
+   </indexterm>
+
+   <para>
+    Session variables are database objects that can hold a value.
+    Session variables, like relations, exist within a schema and their access is
+    controlled via <command>GRANT</command> and <command>REVOKE</command> commands.
+    A session variable can be created by the <command>CREATE VARIABLE</command> command.
+   </para>
+
+   <para>
+    The value of a session variable is set with the <command>LET</command> SQL command.
+    While session variables share properties with tables, their value cannot be updated
+    with an <command>UPDATE</command> command. The value of a session variable may be
+    retrieved by the <command>SELECT</command> SQL command.
+<programlisting>
+CREATE VARIABLE var1 AS date;
+LET var1 = current_date;
+SELECT var1;
+</programlisting>
+
+    or
+
+<programlisting>
+CREATE VARIABLE public.current_user_id AS integer;
+GRANT READ ON VARIABLE public.current_user_id TO PUBLIC;
+LET current_user_id = (SELECT id FROM users WHERE usename = session_user);
+SELECT current_user_id;
+</programlisting>
+   </para>
+
+   <para>
+    The value of a session variable is local to the current session. Retrieving
+    a variable's value returns either a <literal>NULL</literal> or a default value,
+    unless its value is set to something else in the current session using the
+    <command>LET</command> command. The content of a variable is not transactional.
+    This is the same as regular variables in PL languages.
+   </para>
+
+   <para>
+    The session variables can be shadowed by column references in a query. When a
+    query contains identifiers or qualified identifiers that could be used as both
+    a session variable identifiers and as column identifier, then the column
+    identifier is preferred every time. Warnings can be emitted when this situation
+    happens by enabling configuration parameter <xref
+    linkend="guc-session-variables-ambiguity-warning"/>.
+   </para>
+  </sect1>
+
+
   <sect1 id="tutorial-conclusion">
    <title>Conclusion</title>
 
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 1e65c426b2..db805fe079 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -364,6 +364,11 @@
       <entry><link linkend="catalog-pg-user-mapping"><structname>pg_user_mapping</structname></link></entry>
       <entry>mappings of users to foreign servers</entry>
      </row>
+
+     <row>
+      <entry><link linkend="catalog-pg-variable"><structname>pg_variable</structname></link></entry>
+      <entry>session variables</entry>
+     </row>
     </tbody>
    </tgroup>
   </table>
@@ -13988,4 +13993,143 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
 
  </sect1>
 
+ <sect1 id="catalog-pg-variable">
+  <title><structname>pg_variable</structname></title>
+
+  <indexterm zone="catalog-pg-variable">
+   <primary>pg_variable</primary>
+  </indexterm>
+
+  <para>
+   The table <structname>pg_variable</structname> provides information
+   about session variables.
+  </para>
+
+  <table>
+   <title><structname>pg_variable</structname> Columns</title>
+
+   <tgroup cols="4">
+    <thead>
+     <row>
+      <entry>Name</entry>
+      <entry>Type</entry>
+      <entry>References</entry>
+      <entry>Description</entry>
+     </row>
+    </thead>
+    <tbody>
+     <row>
+      <entry><structfield>oid</structfield></entry>
+      <entry><type>oid</type></entry>
+      <entry></entry>
+      <entry>Row identifier</entry>
+     </row>
+
+     <row>
+      <entry><structfield>varname</structfield></entry>
+      <entry><type>name</type></entry>
+      <entry></entry>
+      <entry>Name of the session variable</entry>
+     </row>
+
+     <row>
+      <entry><structfield>varnamespace</structfield></entry>
+      <entry><type>oid</type></entry>
+      <entry><literal><link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.oid</literal></entry>
+      <entry>
+       The OID of the namespace that contains this variable
+      </entry>
+     </row>
+
+     <row>
+      <entry><structfield>vartype</structfield></entry>
+      <entry><type>oid</type></entry>
+      <entry><literal><link linkend="catalog-pg-type"><structname>pg_type</structname></link>.oid</literal></entry>
+      <entry>
+       The OID of the variable's data type.
+      </entry>
+     </row>
+
+     <row>
+      <entry><structfield>vartypmod</structfield></entry>
+      <entry><type>int4</type></entry>
+      <entry></entry>
+      <entry>
+       <structfield>vartypmod</structfield> records type-specific data
+       supplied at variable creation time (for example, the maximum
+       length of a <type>varchar</type> column).  It is passed to
+       type-specific input functions and length coercion functions.
+       The value will generally be -1 for types that do not need <structfield>vartypmod</structfield>.
+      </entry>
+     </row>
+
+     <row>
+      <entry><structfield>varowner</structfield></entry>
+      <entry><type>oid</type></entry>
+      <entry><literal><link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.oid</literal></entry>
+      <entry>Owner of the variable</entry>
+     </row>
+
+     <row>
+      <entry><structfield>varcollation</structfield></entry>
+      <entry><type>oid</type></entry>
+      <entry><literal><link linkend="catalog-pg-collation"><structname>pg_collation</structname></link>.oid</literal></entry>
+      <entry>
+       The defined collation of the variable, or zero if the variable is
+       not of a collatable data type.
+      </entry>
+     </row>
+
+     <row>
+      <entry><structfield>varisnotnull</structfield></entry>
+      <entry><type>boolean</type></entry>
+      <entry></entry>
+      <entry>
+       True if the session variable doesn't allow null value. The default value is false.
+      </entry>
+     </row>
+
+     <row>
+      <entry><structfield>varisimmutable</structfield></entry>
+      <entry><type>boolean</type></entry>
+      <entry></entry>
+      <entry>
+       True if the variable is immutable (cannot be modified). The default value is false.
+      </entry>
+     </row>
+
+     <row>
+      <entry><structfield>vareoxaction</structfield></entry>
+      <entry><type>char</type></entry>
+      <entry></entry>
+      <entry>
+       Action performed at end of transaction:
+       <literal>n</literal> = no action, <literal>d</literal> = drop the variable,
+       <literal>r</literal> = reset the variable to its default value.
+      </entry>
+     </row>
+
+     <row>
+      <entry><structfield>vardefexpr</structfield></entry>
+      <entry><type>pg_node_tree</type></entry>
+      <entry></entry>
+      <entry>The internal representation of the variable default value.</entry>
+     </row>
+
+     <row>
+      <entry><structfield>varacl</structfield></entry>
+      <entry><type>aclitem[]</type></entry>
+      <entry></entry>
+      <entry>
+       Access privileges; see
+       <xref linkend="sql-grant"/> and
+       <xref linkend="sql-revoke"/>
+       for details
+      </entry>
+     </row>
+    </tbody>
+   </tgroup>
+  </table>
+ </sect1>
+
 </chapter>
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 692d8a2a17..8990337f81 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -10023,6 +10023,21 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-session-variables-ambiguity-warning" xreflabel="session_variables_ambiguity_warning">
+      <term><varname>session_variables_ambiguity_warning</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>session_variables_ambiguity_warning</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        When on, a warning is raised when any identifier in a query could be used as both
+        a column identifier, routine variable or a session variable identifier.
+        The default is <literal>off</literal>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-standard-conforming-strings" xreflabel="standard_conforming_strings">
       <term><varname>standard_conforming_strings</varname> (<type>boolean</type>)
       <indexterm><primary>strings</primary><secondary>standard conforming</secondary></indexterm>
diff --git a/doc/src/sgml/glossary.sgml b/doc/src/sgml/glossary.sgml
index 1835d0e65a..641dcc67b3 100644
--- a/doc/src/sgml/glossary.sgml
+++ b/doc/src/sgml/glossary.sgml
@@ -1463,6 +1463,22 @@
    </glossdef>
   </glossentry>
 
+  <glossentry id="glossary-session-variable">
+   <glossterm>Session variable</glossterm>
+   <glossdef>
+    <para>
+     A persistent database object that holds a value in session memory.
+     This memory is not shared across sessions, and after session end, this
+     memory (the value) is released. The access (read or write) to session variables
+     is controlled by access rights similarly to other database object access rights.
+    </para>
+    <para>
+     For more information, see
+     <xref linkend="tutorial-session-variables"/>.
+    </para>
+   </glossdef>
+  </glossentry>
+
   <glossentry id="glossary-shared-memory">
    <glossterm>Shared memory</glossterm>
    <glossdef>
diff --git a/doc/src/sgml/plpgsql.sgml b/doc/src/sgml/plpgsql.sgml
index e5c1356d8c..423677b9af 100644
--- a/doc/src/sgml/plpgsql.sgml
+++ b/doc/src/sgml/plpgsql.sgml
@@ -5952,6 +5952,19 @@ $$ LANGUAGE plpgsql STRICT IMMUTABLE;
 </programlisting>
     </para>
    </sect3>
+
+   <sect3>
+    <title><command>Session variables and constants</command></title>
+
+    <para>
+     The <application>PL/pgSQL</application> language has no packages,
+     and therefore no package variables or package constants.
+     <productname>PostgreSQL</productname> has session variables and
+     immutable session variables. Session variables can be created
+     by <command>CREATE VARIABLE</command> described in <xref
+     linkend="sql-createvariable"/>.
+    </para>
+   </sect3>
   </sect2>
 
   <sect2 id="plpgsql-porting-appendix">
diff --git a/doc/src/sgml/ref/allfiles.sgml b/doc/src/sgml/ref/allfiles.sgml
index d67270ccc3..8458cad752 100644
--- a/doc/src/sgml/ref/allfiles.sgml
+++ b/doc/src/sgml/ref/allfiles.sgml
@@ -47,6 +47,7 @@ Complete list of usable sgml source files in this directory.
 <!ENTITY alterType          SYSTEM "alter_type.sgml">
 <!ENTITY alterUser          SYSTEM "alter_user.sgml">
 <!ENTITY alterUserMapping   SYSTEM "alter_user_mapping.sgml">
+<!ENTITY alterVariable      SYSTEM "alter_variable.sgml">
 <!ENTITY alterView          SYSTEM "alter_view.sgml">
 <!ENTITY analyze            SYSTEM "analyze.sgml">
 <!ENTITY begin              SYSTEM "begin.sgml">
@@ -99,6 +100,7 @@ Complete list of usable sgml source files in this directory.
 <!ENTITY createType         SYSTEM "create_type.sgml">
 <!ENTITY createUser         SYSTEM "create_user.sgml">
 <!ENTITY createUserMapping  SYSTEM "create_user_mapping.sgml">
+<!ENTITY createVariable     SYSTEM "create_variable.sgml">
 <!ENTITY createView         SYSTEM "create_view.sgml">
 <!ENTITY deallocate         SYSTEM "deallocate.sgml">
 <!ENTITY declare            SYSTEM "declare.sgml">
@@ -148,6 +150,7 @@ Complete list of usable sgml source files in this directory.
 <!ENTITY dropUser           SYSTEM "drop_user.sgml">
 <!ENTITY dropUserMapping    SYSTEM "drop_user_mapping.sgml">
 <!ENTITY dropView           SYSTEM "drop_view.sgml">
+<!ENTITY dropVariable       SYSTEM "drop_variable.sgml">
 <!ENTITY end                SYSTEM "end.sgml">
 <!ENTITY execute            SYSTEM "execute.sgml">
 <!ENTITY explain            SYSTEM "explain.sgml">
@@ -155,6 +158,7 @@ Complete list of usable sgml source files in this directory.
 <!ENTITY grant              SYSTEM "grant.sgml">
 <!ENTITY importForeignSchema SYSTEM "import_foreign_schema.sgml">
 <!ENTITY insert             SYSTEM "insert.sgml">
+<!ENTITY let                SYSTEM "let.sgml">
 <!ENTITY listen             SYSTEM "listen.sgml">
 <!ENTITY load               SYSTEM "load.sgml">
 <!ENTITY lock               SYSTEM "lock.sgml">
diff --git a/doc/src/sgml/ref/alter_default_privileges.sgml b/doc/src/sgml/ref/alter_default_privileges.sgml
index f1d54f5aa3..7eb1e8cf8b 100644
--- a/doc/src/sgml/ref/alter_default_privileges.sgml
+++ b/doc/src/sgml/ref/alter_default_privileges.sgml
@@ -50,6 +50,10 @@ GRANT { USAGE | CREATE | ALL [ PRIVILEGES ] }
     ON SCHEMAS
     TO { [ GROUP ] <replaceable class="parameter">role_name</replaceable> | PUBLIC } [, ...] [ WITH GRANT OPTION ]
 
+GRANT { READ | WRITE | ALL [ PRIVILEGES ] }
+    ON VARIABLES
+    TO { [ GROUP ] <replaceable class="parameter">role_name</replaceable> | PUBLIC } [, ...] [ WITH GRANT OPTION ]
+
 REVOKE [ GRANT OPTION FOR ]
     { { SELECT | INSERT | UPDATE | DELETE | TRUNCATE | REFERENCES | TRIGGER }
     [, ...] | ALL [ PRIVILEGES ] }
@@ -81,6 +85,12 @@ REVOKE [ GRANT OPTION FOR ]
     ON SCHEMAS
     FROM { [ GROUP ] <replaceable class="parameter">role_name</replaceable> | PUBLIC } [, ...]
     [ CASCADE | RESTRICT ]
+
+REVOKE [ GRANT OPTION FOR ]
+    { { READ | WRITE } [, ...] | ALL [ PRIVILEGES ] }
+    ON VARIABLES
+    FROM { [ GROUP ] <replaceable class="parameter">role_name</replaceable> | PUBLIC } [, ...]
+    [ CASCADE | RESTRICT ]
 </synopsis>
  </refsynopsisdiv>
 
@@ -92,8 +102,8 @@ REVOKE [ GRANT OPTION FOR ]
    that will be applied to objects created in the future.  (It does not
    affect privileges assigned to already-existing objects.)  Currently,
    only the privileges for schemas, tables (including views and foreign
-   tables), sequences, functions, and types (including domains) can be
-   altered.  For this command, functions include aggregates and procedures.
+   tables), sequences, functions, types (including domains) and session variables
+   can be altered.  For this command, functions include aggregates and procedures.
    The words <literal>FUNCTIONS</literal> and <literal>ROUTINES</literal> are
    equivalent in this command.  (<literal>ROUTINES</literal> is preferred
    going forward as the standard term for functions and procedures taken
diff --git a/doc/src/sgml/ref/alter_variable.sgml b/doc/src/sgml/ref/alter_variable.sgml
new file mode 100644
index 0000000000..50b1f6a2d7
--- /dev/null
+++ b/doc/src/sgml/ref/alter_variable.sgml
@@ -0,0 +1,179 @@
+<!--
+doc/src/sgml/ref/alter_variable.sgml
+PostgreSQL documentation
+-->
+
+<refentry id="sql-altervariable">
+ <indexterm zone="sql-altervariable">
+  <primary>ALTER VARIABLE</primary>
+ </indexterm>
+
+ <indexterm>
+  <primary>session variable</primary>
+  <secondary>altering</secondary>
+ </indexterm>
+
+ <refmeta>
+  <refentrytitle>ALTER VARIABLE</refentrytitle>
+  <manvolnum>7</manvolnum>
+  <refmiscinfo>SQL - Language Statements</refmiscinfo>
+ </refmeta>
+
+ <refnamediv>
+  <refname>ALTER VARIABLE</refname>
+  <refpurpose>
+   change the definition of a variable
+  </refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+<synopsis>
+ALTER VARIABLE <replaceable class="parameter">name</replaceable> OWNER TO { <replaceable class="parameter">new_owner</replaceable> | CURRENT_USER | SESSION_USER }
+ALTER VARIABLE <replaceable class="parameter">name</replaceable> RENAME TO <replaceable class="parameter">new_name</replaceable>
+ALTER VARIABLE <replaceable class="parameter">name</replaceable> SET SCHEMA <replaceable class="parameter">new_schema</replaceable>
+</synopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+  <title>Description</title>
+
+  <para>
+   The <command>ALTER VARIABLE</command> command changes the definition of an existing
+   variable. There are several subforms:
+
+  <variablelist>
+   <varlistentry>
+    <term><literal>OWNER</literal></term>
+    <listitem>
+     <para>
+      This form changes the owner of the variable.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>RENAME</literal></term>
+    <listitem>
+     <para>
+      This form changes the name of the variable.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>SET SCHEMA</literal></term>
+    <listitem>
+     <para>
+      This form moves the variable into another schema.
+     </para>
+    </listitem>
+   </varlistentry>
+
+  </variablelist>
+  </para>
+
+  <para>
+   Only the owner or a superuser is allowed to alter a session variable.
+   In order to move a session variable from one schema to another, the user must
+   also have the <literal>CREATE</literal> privilege on the new schema (or be
+   a superuser).
+
+   In order to move the session variable ownership from one role to another,
+   the user must also be a direct or indirect member of the new
+   owning role, and that role must have the <literal>CREATE</literal> privilege
+   on the variable's schema (or be a superuser). These restrictions enforce that
+   altering the owner doesn't do anything you couldn't do by dropping and
+   recreating the variable.
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>Parameters</title>
+
+   <para>
+    <variablelist>
+     <varlistentry>
+      <term><replaceable class="parameter">name</replaceable></term>
+      <listitem>
+       <para>
+        The name (possibly schema-qualified) of the existing variable to
+        alter.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term><replaceable class="parameter">new_name</replaceable></term>
+      <listitem>
+       <para>
+        The new name for the variable.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term><replaceable class="parameter">new_owner</replaceable></term>
+      <listitem>
+       <para>
+        The user name of the new owner of the variable.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term><replaceable class="parameter">new_schema</replaceable></term>
+      <listitem>
+       <para>
+        The new schema for the variable.
+       </para>
+      </listitem>
+     </varlistentry>
+    </variablelist>
+   </para>
+  </refsect1>
+
+ <refsect1>
+  <title>Examples</title>
+
+  <para>
+   To rename a variable:
+<programlisting>
+ALTER VARIABLE foo RENAME TO boo;
+</programlisting>
+  </para>
+
+  <para>
+   To change the owner of the variable <literal>boo</literal>
+   to <literal>joe</literal>:
+<programlisting>
+ALTER VARIABLE boo OWNER TO joe;
+</programlisting>
+  </para>
+
+  <para>
+   To change the schema of the variable <literal>boo</literal>
+   to <literal>private</literal>:
+<programlisting>
+ALTER VARIABLE boo SET SCHEMA private;
+</programlisting>
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>Compatibility</title>
+
+  <para>
+   Session variables and this command in particular are a PostgreSQL extension.
+  </para>
+ </refsect1>
+
+ <refsect1 id="sql-altervariable-see-also">
+  <title>See Also</title>
+
+  <simplelist type="inline">
+   <member><xref linkend="sql-createvariable"/></member>
+   <member><xref linkend="sql-dropvariable"/></member>
+   <member><xref linkend="sql-let"/></member>
+  </simplelist>
+ </refsect1>
+</refentry>
diff --git a/doc/src/sgml/ref/comment.sgml b/doc/src/sgml/ref/comment.sgml
index b12796095f..0e37570e0c 100644
--- a/doc/src/sgml/ref/comment.sgml
+++ b/doc/src/sgml/ref/comment.sgml
@@ -65,6 +65,7 @@ COMMENT ON
   TRANSFORM FOR <replaceable>type_name</replaceable> LANGUAGE <replaceable>lang_name</replaceable> |
   TRIGGER <replaceable class="parameter">trigger_name</replaceable> ON <replaceable class="parameter">table_name</replaceable> |
   TYPE <replaceable class="parameter">object_name</replaceable> |
+  VARIABLE <replaceable class="parameter">object_name</replaceable> |
   VIEW <replaceable class="parameter">object_name</replaceable>
 } IS '<replaceable class="parameter">text</replaceable>'
 
diff --git a/doc/src/sgml/ref/create_variable.sgml b/doc/src/sgml/ref/create_variable.sgml
new file mode 100644
index 0000000000..38b8a2fbc6
--- /dev/null
+++ b/doc/src/sgml/ref/create_variable.sgml
@@ -0,0 +1,193 @@
+<!--
+doc/src/sgml/ref/create_variable.sgml
+PostgreSQL documentation
+-->
+
+<refentry id="sql-createvariable">
+ <indexterm zone="sql-createvariable">
+  <primary>CREATE VARIABLE</primary>
+ </indexterm>
+
+ <indexterm>
+  <primary>session variable</primary>
+  <secondary>defining</secondary>
+ </indexterm>
+
+ <refmeta>
+  <refentrytitle>CREATE VARIABLE</refentrytitle>
+  <manvolnum>7</manvolnum>
+  <refmiscinfo>SQL - Language Statements</refmiscinfo>
+ </refmeta>
+
+ <refnamediv>
+  <refname>CREATE VARIABLE</refname>
+  <refpurpose>define a session variable</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+<synopsis>
+CREATE [ { TEMPORARY | TEMP } ] [ IMMUTABLE ] VARIABLE [ IF NOT EXISTS ] <replaceable class="parameter">name</replaceable> [ AS ] <replaceable class="parameter">data_type</replaceable> ] [ COLLATE <replaceable class="parameter">collation</replaceable> ]
+    [ NOT NULL ] [ DEFAULT <replaceable class="parameter">default_expr</replaceable> ] [ { ON COMMIT DROP | ON TRANSACTION END RESET } ]
+</synopsis>
+ </refsynopsisdiv>
+ <refsect1>
+  <title>Description</title>
+
+  <para>
+   The <command>CREATE VARIABLE</command> command creates a session variable.
+   Session variables, like relations, exist within a schema and their access is
+   controlled via <command>GRANT</command> and <command>REVOKE</command> commands.
+   Changing a session variable is non-transactional.
+  </para>
+
+  <para>
+   The value of a session variable is local to the current session. Retrieving
+   a variable's value returns either a NULL or a default value, unless its value
+   is set to something else in the current session with a LET command. The content
+   of a variable is not transactional. This is the same as regular variables in PL languages.
+  </para>
+
+  <para>
+   Session variables are retrieved by the <command>SELECT</command> SQL command.
+   Their value is set with the <command>LET</command> SQL command.
+   While session variables share properties with tables, their value cannot be updated
+   with an <command>UPDATE</command> command.
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>Parameters</title>
+
+  <variablelist>
+   <varlistentry>
+    <term><literal>IMMUTABLE</literal></term>
+    <listitem>
+     <para>
+      The assigned value of the variable cannot be changed. The value can be assigned
+      by command <command>LET</command> only first time (when session variable has not
+      defined default expression), or never (when session variable has defined not null
+      default expression).
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>IF NOT EXISTS</literal></term>
+    <listitem>
+     <para>
+      Do not throw an error if the name already exists. A notice is issued in this case.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><replaceable class="parameter">name</replaceable></term>
+    <listitem>
+     <para>
+      The name, optionally schema-qualified, of the variable.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><replaceable class="parameter">data_type</replaceable></term>
+    <listitem>
+     <para>
+      The name, optionally schema-qualified, of the data type of the variable.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>COLLATE <replaceable>collation</replaceable></literal></term>
+    <listitem>
+     <para>
+      The <literal>COLLATE</literal> clause assigns a collation to
+      the variable (which must be of a collatable data type).
+      If not specified, the data type's default collation is used.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>NOT NULL</literal></term>
+    <listitem>
+     <para>
+      The <literal>NOT NULL</literal> clause forbids setting the variable to
+      a null value. A variable created as NOT NULL and without an explicitly
+      declared default value cannot be read until it is initialized by a LET
+      command. This requires the user to explicitly initialize the variable
+      content before reading it.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>DEFAULT <replaceable>default_expr</replaceable></literal></term>
+    <listitem>
+     <para>
+      The <literal>DEFAULT</literal> clause can be used to assign a default value to
+      a session variable.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>ON COMMIT DROP</literal>, <literal>ON TRANSACTION END RESET</literal></term>
+    <listitem>
+     <para>
+      The <literal>ON COMMIT DROP</literal> clause specifies the behaviour
+      of a temporary session variable at transaction commit. With this clause, the
+      variable is dropped at commit time. The clause is only allowed
+      for temporary variables. The <literal>ON TRANSACTION END RESET</literal>
+      clause causes the variable to be reset to its default value when
+      the transaction is committed or rolled back.
+     </para>
+    </listitem>
+   </varlistentry>
+
+  </variablelist>
+ </refsect1>
+
+ <refsect1>
+  <title>Notes</title>
+
+  <para>
+   Use the <command>DROP VARIABLE</command> command to remove a variable.
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>Examples</title>
+
+  <para>
+   Create an integer variable <literal>var1</literal>:
+<programlisting>
+CREATE VARIABLE var1 AS date;
+LET var1 = current_date;
+SELECT var1;
+</programlisting>
+  </para>
+
+ </refsect1>
+
+ <refsect1>
+  <title>Compatibility</title>
+
+  <para>
+   The <command>CREATE VARIABLE</command> command is a PostgreSQL extension.
+   <!-- The choice of wording here seems to be left to personal preference... -->
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>See Also</title>
+
+  <simplelist type="inline">
+   <member><xref linkend="sql-altervariable"/></member>
+   <member><xref linkend="sql-dropvariable"/></member>
+   <member><xref linkend="sql-let"/></member>
+  </simplelist>
+ </refsect1>
+
+</refentry>
diff --git a/doc/src/sgml/ref/discard.sgml b/doc/src/sgml/ref/discard.sgml
index bf44c523ca..48d96af187 100644
--- a/doc/src/sgml/ref/discard.sgml
+++ b/doc/src/sgml/ref/discard.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-DISCARD { ALL | PLANS | SEQUENCES | TEMPORARY | TEMP }
+DISCARD { ALL | PLANS | SEQUENCES | TEMPORARY | TEMP | VARIABLES }
 </synopsis>
  </refsynopsisdiv>
 
@@ -75,6 +75,17 @@ DISCARD { ALL | PLANS | SEQUENCES | TEMPORARY | TEMP }
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>VARIABLES</literal></term>
+    <listitem>
+     <para>
+      Resets the value of all session variables. If a variable
+      is later reused, it is re-initialized to either
+      <literal>NULL</literal> or its default value.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>ALL</literal></term>
     <listitem>
@@ -93,6 +104,7 @@ SELECT pg_advisory_unlock_all();
 DISCARD PLANS;
 DISCARD TEMP;
 DISCARD SEQUENCES;
+DISCARD VARIABLES;
 </programlisting></para>
     </listitem>
    </varlistentry>
diff --git a/doc/src/sgml/ref/drop_variable.sgml b/doc/src/sgml/ref/drop_variable.sgml
new file mode 100644
index 0000000000..5cced4ea90
--- /dev/null
+++ b/doc/src/sgml/ref/drop_variable.sgml
@@ -0,0 +1,120 @@
+<!--
+doc/src/sgml/ref/drop_variable.sgml
+PostgreSQL documentation
+-->
+
+<refentry id="sql-dropvariable">
+ <indexterm zone="sql-dropvariable">
+  <primary>DROP VARIABLE</primary>
+ </indexterm>
+
+ <indexterm>
+  <primary>session variable</primary>
+  <secondary>removing</secondary>
+ </indexterm>
+
+ <refmeta>
+  <refentrytitle>DROP VARIABLE</refentrytitle>
+  <manvolnum>7</manvolnum>
+  <refmiscinfo>SQL - Language Statements</refmiscinfo>
+ </refmeta>
+
+ <refnamediv>
+  <refname>DROP VARIABLE</refname>
+  <refpurpose>remove a session variable</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+<synopsis>
+DROP VARIABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable> [, ...] [ CASCADE | RESTRICT ]
+</synopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+  <title>Description</title>
+
+  <para>
+   <command>DROP VARIABLE</command> removes a session variable.
+   A variable can only be dropped by its owner or a superuser.
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>Parameters</title>
+
+  <variablelist>
+   <varlistentry>
+    <term><literal>IF EXISTS</literal></term>
+    <listitem>
+     <para>
+      Do not throw an error if the variable does not exist. A notice is issued
+      in this case.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><replaceable class="parameter">name</replaceable></term>
+    <listitem>
+     <para>
+      The name, optionally schema-qualified, of a session variable.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>CASCADE</literal></term>
+    <listitem>
+     <para>
+      Automatically drop objects that depend on the variable (such as
+      views),
+      and in turn all objects that depend on those objects
+      (see <xref linkend="ddl-depend"/>).
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>RESTRICT</literal></term>
+    <listitem>
+     <para>
+      Refuse to drop the variable if any objects depend on it.  This is
+      the default.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+ </refsect1>
+
+ <refsect1>
+  <title>Examples</title>
+
+  <para>
+   To remove the session variable <literal>var1</literal>:
+
+<programlisting>
+DROP VARIABLE var1;
+</programlisting></para>
+ </refsect1>
+
+ <refsect1>
+  <title>Compatibility</title>
+
+  <para>
+   The <command>DROP VARIABLE</command> command is a PostgreSQL extension.
+   <!-- create variable is a "PostgreSQL feature",
+        this is a "proprietary PostgreSQL command" ... -->
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>See Also</title>
+
+  <simplelist type="inline">
+   <member><xref linkend="sql-altervariable"/></member>
+   <member><xref linkend="sql-createvariable"/></member>
+   <member><xref linkend="sql-let"/></member>
+  </simplelist>
+ </refsect1>
+
+</refentry>
diff --git a/doc/src/sgml/ref/grant.sgml b/doc/src/sgml/ref/grant.sgml
index a897712de2..74adf80331 100644
--- a/doc/src/sgml/ref/grant.sgml
+++ b/doc/src/sgml/ref/grant.sgml
@@ -103,6 +103,11 @@ GRANT <replaceable class="parameter">role_name</replaceable> [, ...] TO <replace
   | CURRENT_ROLE
   | CURRENT_USER
   | SESSION_USER
+
+GRANT { READ | WRITE | ALL [ PRIVILEGES ] }
+    ON VARIABLE <replaceable>variable_name</replaceable> [, ...]
+    TO <replaceable class="parameter">role_specification</replaceable> [, ...] [ WITH GRANT OPTION ]
+
 </synopsis>
  </refsynopsisdiv>
 
@@ -201,6 +206,24 @@ GRANT <replaceable class="parameter">role_name</replaceable> [, ...] TO <replace
      </listitem>
     </varlistentry>
 
+    <varlistentry>
+     <term><literal>READ</literal></term>
+     <listitem>
+      <para>
+       Allows reading of a session variable.
+      </para>
+     </listitem>
+    </varlistentry>
+
+    <varlistentry>
+     <term><literal>WRITE</literal></term>
+     <listitem>
+      <para>
+       Allows setting the value of a session variable.
+      </para>
+     </listitem>
+    </varlistentry>
+
     <varlistentry>
      <term><literal>ALL PRIVILEGES</literal></term>
      <listitem>
diff --git a/doc/src/sgml/ref/let.sgml b/doc/src/sgml/ref/let.sgml
new file mode 100644
index 0000000000..fcbfefb4f6
--- /dev/null
+++ b/doc/src/sgml/ref/let.sgml
@@ -0,0 +1,107 @@
+<!--
+doc/src/sgml/ref/let.sgml
+PostgreSQL documentation
+-->
+
+<refentry id="sql-let">
+ <indexterm zone="sql-let">
+  <primary>LET</primary>
+ </indexterm>
+
+ <indexterm>
+  <primary>session variable</primary>
+  <secondary>changing</secondary>
+ </indexterm>
+
+ <refmeta>
+  <refentrytitle>LET</refentrytitle>
+  <manvolnum>7</manvolnum>
+  <refmiscinfo>SQL - Language Statements</refmiscinfo>
+ </refmeta>
+
+ <refnamediv>
+  <refname>LET</refname>
+  <refpurpose>change a session variable's value</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+<synopsis>
+LET <replaceable class="parameter">session_variable</replaceable> = <replaceable class="parameter">sql_expression</replaceable>
+LET <replaceable class="parameter">session_variable</replaceable> = DEFAULT
+</synopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+  <title>Description</title>
+
+  <para>
+   The <command>LET</command> command assigns a value to the specified session variable.
+  </para>
+
+ </refsect1>
+
+ <refsect1>
+  <title>Parameters</title>
+
+  <variablelist>
+   <varlistentry>
+    <term><literal>session_variable</literal></term>
+    <listitem>
+     <para>
+      The name of the session variable.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>sql_expression</literal></term>
+    <listitem>
+     <para>
+      An SQL expression. The result is cast to the data type of the session variable.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><literal>DEFAULT</literal></term>
+    <listitem>
+     <para>
+      Reset the session variable to its default value, if that is defined.
+      If no explicit default value has been assigned, the session variable
+      is set to NULL.
+     </para>
+    </listitem>
+   </varlistentry>
+  </variablelist>
+
+  <para>
+   Example:
+<programlisting>
+CREATE VARIABLE myvar AS integer;
+LET myvar = 10;
+LET myvar = (SELECT sum(val) FROM tab);
+LET myvar = DEFAULT;
+</programlisting>
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>Compatibility</title>
+
+  <para>
+   <literal>LET</literal> extends the syntax defined in the SQL
+   standard. The <literal>SET</literal> command from the SQL standard
+   is used for different purposes in PostgreSQL.
+  </para>
+ </refsect1>
+
+ <refsect1>
+  <title>See Also</title>
+
+  <simplelist type="inline">
+   <member><xref linkend="sql-altervariable"/></member>
+   <member><xref linkend="sql-createvariable"/></member>
+   <member><xref linkend="sql-dropvariable"/></member>
+  </simplelist>
+ </refsect1>
+</refentry>
diff --git a/doc/src/sgml/ref/pg_restore.sgml b/doc/src/sgml/ref/pg_restore.sgml
index 526986eadb..3e8c26c695 100644
--- a/doc/src/sgml/ref/pg_restore.sgml
+++ b/doc/src/sgml/ref/pg_restore.sgml
@@ -106,6 +106,17 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>-A <replaceable class="parameter">schema_variable</replaceable></option></term>
+      <term><option>--variable=<replaceable class="parameter">schema_variable</replaceable></option></term>
+      <listitem>
+       <para>
+        Restore a named session variable only.  Multiple session variables may be specified with
+        multiple <option>-A</option> switches.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-c</option></term>
       <term><option>--clean</option></term>
diff --git a/doc/src/sgml/ref/revoke.sgml b/doc/src/sgml/ref/revoke.sgml
index 3014c864ea..3e9f9804be 100644
--- a/doc/src/sgml/ref/revoke.sgml
+++ b/doc/src/sgml/ref/revoke.sgml
@@ -130,6 +130,12 @@ REVOKE [ ADMIN OPTION FOR ]
   | CURRENT_ROLE
   | CURRENT_USER
   | SESSION_USER
+
+REVOKE [ GRANT OPTION FOR ]
+    { { READ | WRITE } [, ...] | ALL [ PRIVILEGES ] }
+    ON VARIABLE <replaceable>variable_name</replaceable> [, ...]
+    FROM { [ GROUP ] <replaceable class="parameter">role_name</replaceable> | PUBLIC } [, ...]
+    [ CASCADE | RESTRICT ]
 </synopsis>
  </refsynopsisdiv>
 
diff --git a/doc/src/sgml/ref/set_role.sgml b/doc/src/sgml/ref/set_role.sgml
index f02babf3af..911d17369c 100644
--- a/doc/src/sgml/ref/set_role.sgml
+++ b/doc/src/sgml/ref/set_role.sgml
@@ -98,7 +98,7 @@ RESET ROLE
   </para>
 
   <para>
-   <command>SET ROLE</command> does not process session variables as specified by
+   <command>SET ROLE</command> does not set run-time configuration parameters specified by
    the role's <link linkend="sql-alterrole"><command>ALTER ROLE</command></link> settings;  this only happens during
    login.
   </para>
diff --git a/doc/src/sgml/reference.sgml b/doc/src/sgml/reference.sgml
index da421ff24e..f9e42443b6 100644
--- a/doc/src/sgml/reference.sgml
+++ b/doc/src/sgml/reference.sgml
@@ -75,6 +75,7 @@
    &alterType;
    &alterUser;
    &alterUserMapping;
+   &alterVariable;
    &alterView;
    &analyze;
    &begin;
@@ -127,6 +128,7 @@
    &createType;
    &createUser;
    &createUserMapping;
+   &createVariable;
    &createView;
    &deallocate;
    &declare;
@@ -175,6 +177,7 @@
    &dropType;
    &dropUser;
    &dropUserMapping;
+   &dropVariable;
    &dropView;
    &end;
    &execute;
@@ -183,6 +186,7 @@
    &grant;
    &importForeignSchema;
    &insert;
+   &let;
    &listen;
    &load;
    &lock;
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index c9516e03fa..33d499845b 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -35,6 +35,7 @@
 #include "catalog/pg_enum.h"
 #include "catalog/storage.h"
 #include "commands/async.h"
+#include "commands/session_variable.h"
 #include "commands/tablecmds.h"
 #include "commands/trigger.h"
 #include "common/pg_prng.h"
@@ -2197,6 +2198,9 @@ CommitTransaction(void)
 	 */
 	smgrDoPendingSyncs(true, is_parallel_worker);
 
+	/* Let ON COMMIT DROP or ON TRANSACTION END */
+	AtPreEOXact_SessionVariable_on_xact_actions(true);
+
 	/* close large objects before lower-level cleanup */
 	AtEOXact_LargeObject(true);
 
@@ -2773,6 +2777,9 @@ AbortTransaction(void)
 	AtAbort_Portals();
 	smgrDoPendingSyncs(false, is_parallel_worker);
 	AtEOXact_LargeObject(false);
+
+	/* 'false' means it's abort */
+	AtPreEOXact_SessionVariable_on_xact_actions(false);
 	AtAbort_Notify();
 	AtEOXact_RelationMap(false, is_parallel_worker);
 	AtAbort_Twophase();
@@ -4962,6 +4969,8 @@ CommitSubTransaction(void)
 	AtEOSubXact_SPI(true, s->subTransactionId);
 	AtEOSubXact_on_commit_actions(true, s->subTransactionId,
 								  s->parent->subTransactionId);
+	AtEOSubXact_SessionVariable_on_xact_actions(true, s->subTransactionId,
+												s->parent->subTransactionId);
 	AtEOSubXact_Namespace(true, s->subTransactionId,
 						  s->parent->subTransactionId);
 	AtEOSubXact_Files(true, s->subTransactionId,
@@ -5126,6 +5135,8 @@ AbortSubTransaction(void)
 		AtEOSubXact_SPI(false, s->subTransactionId);
 		AtEOSubXact_on_commit_actions(false, s->subTransactionId,
 									  s->parent->subTransactionId);
+		AtEOSubXact_SessionVariable_on_xact_actions(false, s->subTransactionId,
+													s->parent->subTransactionId);
 		AtEOSubXact_Namespace(false, s->subTransactionId,
 							  s->parent->subTransactionId);
 		AtEOSubXact_Files(false, s->subTransactionId,
diff --git a/src/backend/catalog/Makefile b/src/backend/catalog/Makefile
index df5268fbc3..7792715e8f 100644
--- a/src/backend/catalog/Makefile
+++ b/src/backend/catalog/Makefile
@@ -43,6 +43,7 @@ OBJS = \
 	pg_shdepend.o \
 	pg_subscription.o \
 	pg_type.o \
+	pg_variable.o \
 	storage.o \
 	toasting.o
 
@@ -69,7 +70,8 @@ CATALOG_HEADERS := \
 	pg_default_acl.h pg_init_privs.h pg_seclabel.h pg_shseclabel.h \
 	pg_collation.h pg_partitioned_table.h pg_range.h pg_transform.h \
 	pg_sequence.h pg_publication.h pg_publication_namespace.h \
-	pg_publication_rel.h pg_subscription.h pg_subscription_rel.h
+	pg_publication_rel.h pg_subscription.h pg_subscription_rel.h \
+	pg_variable.h
 
 GENERATED_HEADERS := $(CATALOG_HEADERS:%.h=%_d.h) schemapg.h system_fk_info.h
 
diff --git a/src/backend/catalog/aclchk.c b/src/backend/catalog/aclchk.c
index 1dd03a8e51..52027ef890 100644
--- a/src/backend/catalog/aclchk.c
+++ b/src/backend/catalog/aclchk.c
@@ -58,6 +58,7 @@
 #include "catalog/pg_ts_parser.h"
 #include "catalog/pg_ts_template.h"
 #include "catalog/pg_type.h"
+#include "catalog/pg_variable.h"
 #include "commands/dbcommands.h"
 #include "commands/defrem.h"
 #include "commands/event_trigger.h"
@@ -112,6 +113,7 @@ static void ExecGrant_Largeobject(InternalGrant *grantStmt);
 static void ExecGrant_Namespace(InternalGrant *grantStmt);
 static void ExecGrant_Tablespace(InternalGrant *grantStmt);
 static void ExecGrant_Type(InternalGrant *grantStmt);
+static void ExecGrant_Variable(InternalGrant *grantStmt);
 
 static void SetDefaultACLsInSchemas(InternalDefaultACL *iacls, List *nspnames);
 static void SetDefaultACL(InternalDefaultACL *iacls);
@@ -259,6 +261,9 @@ restrict_and_check_grant(bool is_grant, AclMode avail_goptions, bool all_privs,
 		case OBJECT_TYPE:
 			whole_mask = ACL_ALL_RIGHTS_TYPE;
 			break;
+		case OBJECT_VARIABLE:
+			whole_mask = ACL_ALL_RIGHTS_VARIABLE;
+			break;
 		default:
 			elog(ERROR, "unrecognized object type: %d", objtype);
 			/* not reached, but keep compiler quiet */
@@ -498,6 +503,10 @@ ExecuteGrantStmt(GrantStmt *stmt)
 			all_privileges = ACL_ALL_RIGHTS_FOREIGN_SERVER;
 			errormsg = gettext_noop("invalid privilege type %s for foreign server");
 			break;
+		case OBJECT_VARIABLE:
+			all_privileges = ACL_ALL_RIGHTS_VARIABLE;
+			errormsg = gettext_noop("invalid privilege type %s for session variable");
+			break;
 		default:
 			elog(ERROR, "unrecognized GrantStmt.objtype: %d",
 				 (int) stmt->objtype);
@@ -600,6 +609,9 @@ ExecGrantStmt_oids(InternalGrant *istmt)
 		case OBJECT_TABLESPACE:
 			ExecGrant_Tablespace(istmt);
 			break;
+		case OBJECT_VARIABLE:
+			ExecGrant_Variable(istmt);
+			break;
 		default:
 			elog(ERROR, "unrecognized GrantStmt.objtype: %d",
 				 (int) istmt->objtype);
@@ -759,6 +771,16 @@ objectNamesToOids(ObjectType objtype, List *objnames)
 				objects = lappend_oid(objects, srvid);
 			}
 			break;
+		case OBJECT_VARIABLE:
+			foreach(cell, objnames)
+			{
+				RangeVar   *varvar = (RangeVar *) lfirst(cell);
+				Oid			relOid;
+
+				relOid = LookupVariable(varvar->schemaname, varvar->relname, false);
+				objects = lappend_oid(objects, relOid);
+			}
+			break;
 		default:
 			elog(ERROR, "unrecognized GrantStmt.objtype: %d",
 				 (int) objtype);
@@ -848,6 +870,33 @@ objectsInSchemaToOids(ObjectType objtype, List *nspnames)
 					table_close(rel, AccessShareLock);
 				}
 				break;
+			case OBJECT_VARIABLE:
+				{
+					ScanKeyData key;
+					Relation	rel;
+					TableScanDesc scan;
+					HeapTuple	tuple;
+
+					ScanKeyInit(&key,
+								Anum_pg_variable_varnamespace,
+								BTEqualStrategyNumber, F_OIDEQ,
+								ObjectIdGetDatum(namespaceId));
+
+					rel = table_open(VariableRelationId, AccessShareLock);
+					scan = table_beginscan_catalog(rel, 1, &key);
+
+					while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
+					{
+						Oid			oid = ((Form_pg_proc) GETSTRUCT(tuple))->oid;
+
+						objects = lappend_oid(objects, oid);
+					}
+
+					table_endscan(scan);
+					table_close(rel, AccessShareLock);
+				}
+				break;
+
 			default:
 				/* should not happen */
 				elog(ERROR, "unrecognized GrantStmt.objtype: %d",
@@ -1007,6 +1056,10 @@ ExecAlterDefaultPrivilegesStmt(ParseState *pstate, AlterDefaultPrivilegesStmt *s
 			all_privileges = ACL_ALL_RIGHTS_SCHEMA;
 			errormsg = gettext_noop("invalid privilege type %s for schema");
 			break;
+		case OBJECT_VARIABLE:
+			all_privileges = ACL_ALL_RIGHTS_VARIABLE;
+			errormsg = gettext_noop("invalid privilege type %s for session variable");
+			break;
 		default:
 			elog(ERROR, "unrecognized GrantStmt.objtype: %d",
 				 (int) action->objtype);
@@ -1204,6 +1257,12 @@ SetDefaultACL(InternalDefaultACL *iacls)
 				this_privileges = ACL_ALL_RIGHTS_SCHEMA;
 			break;
 
+		case OBJECT_VARIABLE:
+			objtype = DEFACLOBJ_VARIABLE;
+			if (iacls->all_privs && this_privileges == ACL_NO_RIGHTS)
+				this_privileges = ACL_ALL_RIGHTS_VARIABLE;
+			break;
+
 		default:
 			elog(ERROR, "unrecognized objtype: %d",
 				 (int) iacls->objtype);
@@ -1437,6 +1496,9 @@ RemoveRoleFromObjectACL(Oid roleid, Oid classid, Oid objid)
 			case DEFACLOBJ_NAMESPACE:
 				iacls.objtype = OBJECT_SCHEMA;
 				break;
+			case DEFACLOBJ_VARIABLE:
+				iacls.objtype = OBJECT_VARIABLE;
+				break;
 			default:
 				/* Shouldn't get here */
 				elog(ERROR, "unexpected default ACL type: %d",
@@ -1494,6 +1556,9 @@ RemoveRoleFromObjectACL(Oid roleid, Oid classid, Oid objid)
 			case ForeignDataWrapperRelationId:
 				istmt.objtype = OBJECT_FDW;
 				break;
+			case VariableRelationId:
+				istmt.objtype = OBJECT_VARIABLE;
+				break;
 			default:
 				elog(ERROR, "unexpected object class %u", classid);
 				break;
@@ -3225,6 +3290,129 @@ ExecGrant_Type(InternalGrant *istmt)
 	table_close(relation, RowExclusiveLock);
 }
 
+static void
+ExecGrant_Variable(InternalGrant *istmt)
+{
+	Relation	relation;
+	ListCell   *cell;
+
+	if (istmt->all_privs && istmt->privileges == ACL_NO_RIGHTS)
+		istmt->privileges = ACL_ALL_RIGHTS_VARIABLE;
+
+	relation = table_open(VariableRelationId, RowExclusiveLock);
+
+	foreach(cell, istmt->objects)
+	{
+		Oid			varId = lfirst_oid(cell);
+		Form_pg_variable pg_variable_tuple;
+		Datum		aclDatum;
+		bool		isNull;
+		AclMode		avail_goptions;
+		AclMode		this_privileges;
+		Acl		   *old_acl;
+		Acl		   *new_acl;
+		Oid			grantorId;
+		Oid			ownerId;
+		HeapTuple	tuple;
+		HeapTuple	newtuple;
+		Datum		values[Natts_pg_variable];
+		bool		nulls[Natts_pg_variable];
+		bool		replaces[Natts_pg_variable];
+		int			noldmembers;
+		int			nnewmembers;
+		Oid		   *oldmembers;
+		Oid		   *newmembers;
+
+		tuple = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(varId));
+		if (!HeapTupleIsValid(tuple))
+			elog(ERROR, "cache lookup failed for session variable %u", varId);
+
+		pg_variable_tuple = (Form_pg_variable) GETSTRUCT(tuple);
+
+		/*
+		 * Get owner ID and working copy of existing ACL. If there's no ACL,
+		 * substitute the proper default.
+		 */
+		ownerId = pg_variable_tuple->varowner;
+		aclDatum = SysCacheGetAttr(VARIABLEOID, tuple, Anum_pg_variable_varacl,
+								   &isNull);
+		if (isNull)
+		{
+			old_acl = acldefault(OBJECT_VARIABLE, ownerId);
+			/* There are no old member roles according to the catalogs */
+			noldmembers = 0;
+			oldmembers = NULL;
+		}
+		else
+		{
+			old_acl = DatumGetAclPCopy(aclDatum);
+			/* Get the roles mentioned in the existing ACL */
+			noldmembers = aclmembers(old_acl, &oldmembers);
+		}
+
+		/* Determine ID to do the grant as, and available grant options */
+		select_best_grantor(GetUserId(), istmt->privileges,
+							old_acl, ownerId,
+							&grantorId, &avail_goptions);
+
+		/*
+		 * Restrict the privileges to what we can actually grant, and emit the
+		 * standards-mandated warning and error messages.
+		 */
+		this_privileges =
+			restrict_and_check_grant(istmt->is_grant, avail_goptions,
+									 istmt->all_privs, istmt->privileges,
+									 varId, grantorId, OBJECT_VARIABLE,
+									 NameStr(pg_variable_tuple->varname),
+									 0, NULL);
+
+		/*
+		 * Generate new ACL.
+		 */
+		new_acl = merge_acl_with_grant(old_acl, istmt->is_grant,
+									   istmt->grant_option, istmt->behavior,
+									   istmt->grantees, this_privileges,
+									   grantorId, ownerId);
+
+		/*
+		 * We need the members of both old and new ACLs so we can correct the
+		 * shared dependency information.
+		 */
+		nnewmembers = aclmembers(new_acl, &newmembers);
+
+		/* finished building new ACL value, now insert it */
+		MemSet(values, 0, sizeof(values));
+		MemSet(nulls, false, sizeof(nulls));
+		MemSet(replaces, false, sizeof(replaces));
+
+		replaces[Anum_pg_variable_varacl - 1] = true;
+		values[Anum_pg_variable_varacl - 1] = PointerGetDatum(new_acl);
+
+		newtuple = heap_modify_tuple(tuple, RelationGetDescr(relation), values,
+									 nulls, replaces);
+
+		CatalogTupleUpdate(relation, &newtuple->t_self, newtuple);
+
+		/* Update initial privileges for extensions */
+		recordExtensionInitPriv(varId, VariableRelationId, 0, new_acl);
+
+		/* Update the shared dependency ACL info */
+		updateAclDependencies(VariableRelationId, varId, 0,
+							  ownerId,
+							  noldmembers, oldmembers,
+							  nnewmembers, newmembers);
+
+		ReleaseSysCache(tuple);
+
+		pfree(new_acl);
+
+		/* prevent error when processing duplicate objects */
+		CommandCounterIncrement();
+	}
+
+	table_close(relation, RowExclusiveLock);
+}
+
 
 static AclMode
 string_to_privilege(const char *privname)
@@ -3257,6 +3445,10 @@ string_to_privilege(const char *privname)
 		return ACL_CONNECT;
 	if (strcmp(privname, "rule") == 0)
 		return 0;				/* ignore old RULE privileges */
+	if (strcmp(privname, "read") == 0)
+		return ACL_READ;
+	if (strcmp(privname, "write") == 0)
+		return ACL_WRITE;
 	ereport(ERROR,
 			(errcode(ERRCODE_SYNTAX_ERROR),
 			 errmsg("unrecognized privilege type \"%s\"", privname)));
@@ -3292,6 +3484,10 @@ privilege_to_string(AclMode privilege)
 			return "TEMP";
 		case ACL_CONNECT:
 			return "CONNECT";
+		case ACL_READ:
+			return "READ";
+		case ACL_WRITE:
+			return "WRITE";
 		default:
 			elog(ERROR, "unrecognized privilege: %d", (int) privilege);
 	}
@@ -3415,6 +3611,9 @@ aclcheck_error(AclResult aclerr, ObjectType objtype,
 					case OBJECT_TYPE:
 						msg = gettext_noop("permission denied for type %s");
 						break;
+					case OBJECT_VARIABLE:
+						msg = gettext_noop("permission denied for session variable %s");
+						break;
 					case OBJECT_VIEW:
 						msg = gettext_noop("permission denied for view %s");
 						break;
@@ -3526,6 +3725,9 @@ aclcheck_error(AclResult aclerr, ObjectType objtype,
 					case OBJECT_TYPE:
 						msg = gettext_noop("must be owner of type %s");
 						break;
+					case OBJECT_VARIABLE:
+						msg = gettext_noop("must be owner of session variable %s");
+						break;
 					case OBJECT_VIEW:
 						msg = gettext_noop("must be owner of view %s");
 						break;
@@ -3671,6 +3873,8 @@ pg_aclmask(ObjectType objtype, Oid table_oid, AttrNumber attnum, Oid roleid,
 			return ACL_NO_RIGHTS;
 		case OBJECT_TYPE:
 			return pg_type_aclmask(table_oid, roleid, mask, how);
+		case OBJECT_VARIABLE:
+			return pg_variable_aclmask(table_oid, roleid, mask, how);
 		default:
 			elog(ERROR, "unrecognized objtype: %d",
 				 (int) objtype);
@@ -4539,6 +4743,66 @@ pg_type_aclmask(Oid type_oid, Oid roleid, AclMode mask, AclMaskHow how)
 	return result;
 }
 
+/*
+ * Exported routine for examining a user's privileges for a variable.
+ */
+AclMode
+pg_variable_aclmask(Oid var_oid, Oid roleid, AclMode mask, AclMaskHow how)
+{
+	AclMode		result;
+	HeapTuple	tuple;
+	Datum		aclDatum;
+	bool		isNull;
+	Acl		   *acl;
+	Oid			ownerId;
+
+	Form_pg_variable varForm;
+
+	/* Bypass permission checks for superusers */
+	if (superuser_arg(roleid))
+		return mask;
+
+	/*
+	 * Must get the variables's tuple from pg_variable
+	 */
+	tuple = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(var_oid));
+	if (!HeapTupleIsValid(tuple))
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_OBJECT),
+				 errmsg("variable with OID %u does not exist",
+						var_oid)));
+	varForm = (Form_pg_variable) GETSTRUCT(tuple);
+
+	/*
+	 * Now get the variable's owner and ACL from the tuple
+	 */
+	ownerId = varForm->varowner;
+
+	aclDatum = SysCacheGetAttr(VARIABLEOID, tuple,
+							   Anum_pg_variable_varacl, &isNull);
+	if (isNull)
+	{
+		/* No ACL, so build default ACL */
+		acl = acldefault(OBJECT_VARIABLE, ownerId);
+		aclDatum = (Datum) 0;
+	}
+	else
+	{
+		/* detoast rel's ACL if necessary */
+		acl = DatumGetAclP(aclDatum);
+	}
+
+	result = aclmask(acl, roleid, ownerId, mask, how);
+
+	/* if we have a detoasted copy, free it */
+	if (acl && (Pointer) acl != DatumGetPointer(aclDatum))
+		pfree(acl);
+
+	ReleaseSysCache(tuple);
+
+	return result;
+}
+
 /*
  * Exported routine for checking a user's access privileges to a column
  *
@@ -4813,6 +5077,18 @@ pg_type_aclcheck(Oid type_oid, Oid roleid, AclMode mode)
 		return ACLCHECK_NO_PRIV;
 }
 
+/*
+ * Exported routine for checking a user's access privileges to a variable
+ */
+AclResult
+pg_variable_aclcheck(Oid type_oid, Oid roleid, AclMode mode)
+{
+	if (pg_variable_aclmask(type_oid, roleid, mode, ACLMASK_ANY) != 0)
+		return ACLCHECK_OK;
+	else
+		return ACLCHECK_NO_PRIV;
+}
+
 /*
  * Ownership check for a relation (specified by OID).
  */
@@ -5430,6 +5706,33 @@ pg_statistics_object_ownercheck(Oid stat_oid, Oid roleid)
 	return has_privs_of_role(roleid, ownerId);
 }
 
+/*
+ * Ownership check for a session variable (specified by OID).
+ */
+bool
+pg_variable_ownercheck(Oid db_oid, Oid roleid)
+{
+	HeapTuple	tuple;
+	Oid			ownerId;
+
+	/* Superusers bypass all permission checking. */
+	if (superuser_arg(roleid))
+		return true;
+
+	tuple = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(db_oid));
+	if (!HeapTupleIsValid(tuple))
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_DATABASE),
+				 errmsg("session variable with OID %u does not exist", db_oid)));
+
+	ownerId = ((Form_pg_variable) GETSTRUCT(tuple))->varowner;
+
+	ReleaseSysCache(tuple);
+
+	return has_privs_of_role(roleid, ownerId);
+}
+
+
 /*
  * Check whether specified role has CREATEROLE privilege (or is a superuser)
  *
@@ -5558,6 +5861,10 @@ get_user_default_acl(ObjectType objtype, Oid ownerId, Oid nsp_oid)
 			defaclobjtype = DEFACLOBJ_NAMESPACE;
 			break;
 
+		case OBJECT_VARIABLE:
+			defaclobjtype = DEFACLOBJ_VARIABLE;
+			break;
+
 		default:
 			return NULL;
 	}
diff --git a/src/backend/catalog/dependency.c b/src/backend/catalog/dependency.c
index ab9e42d7d1..0ba880da62 100644
--- a/src/backend/catalog/dependency.c
+++ b/src/backend/catalog/dependency.c
@@ -63,12 +63,15 @@
 #include "catalog/pg_ts_template.h"
 #include "catalog/pg_type.h"
 #include "catalog/pg_user_mapping.h"
+#include "catalog/pg_variable.h"
 #include "commands/comment.h"
 #include "commands/defrem.h"
 #include "commands/event_trigger.h"
 #include "commands/extension.h"
 #include "commands/policy.h"
 #include "commands/publicationcmds.h"
+#include "commands/schemacmds.h"
+#include "commands/session_variable.h"
 #include "commands/seclabel.h"
 #include "commands/sequence.h"
 #include "commands/trigger.h"
@@ -183,7 +186,8 @@ static const Oid object_classes[] = {
 	PublicationRelationId,		/* OCLASS_PUBLICATION */
 	PublicationRelRelationId,	/* OCLASS_PUBLICATION_REL */
 	SubscriptionRelationId,		/* OCLASS_SUBSCRIPTION */
-	TransformRelationId			/* OCLASS_TRANSFORM */
+	TransformRelationId,		/* OCLASS_TRANSFORM */
+	VariableRelationId			/* OCLASS_VARIABLE */
 };
 
 
@@ -1501,6 +1505,10 @@ doDeletion(const ObjectAddress *object, int flags)
 			DropObjectById(object);
 			break;
 
+		case OCLASS_VARIABLE:
+			RemoveSessionVariable(object->objectId);
+			break;
+
 			/*
 			 * These global object types are not supported here.
 			 */
@@ -1879,6 +1887,11 @@ find_expr_references_walker(Node *node,
 	{
 		Param	   *param = (Param *) node;
 
+		/* A variable parameter depends on the session variable */
+		if (param->paramkind == PARAM_VARIABLE)
+			add_object_address(OCLASS_VARIABLE, param->paramvarid, 0,
+							   context->addrs);
+
 		/* A parameter must depend on the parameter's datatype */
 		add_object_address(OCLASS_TYPE, param->paramtype, 0,
 						   context->addrs);
@@ -2879,6 +2892,9 @@ getObjectClass(const ObjectAddress *object)
 
 		case TransformRelationId:
 			return OCLASS_TRANSFORM;
+
+		case VariableRelationId:
+			return OCLASS_VARIABLE;
 	}
 
 	/* shouldn't get here */
diff --git a/src/backend/catalog/namespace.c b/src/backend/catalog/namespace.c
index 5dbac9c437..fc0e08be46 100644
--- a/src/backend/catalog/namespace.c
+++ b/src/backend/catalog/namespace.c
@@ -39,6 +39,7 @@
 #include "catalog/pg_ts_parser.h"
 #include "catalog/pg_ts_template.h"
 #include "catalog/pg_type.h"
+#include "catalog/pg_variable.h"
 #include "commands/dbcommands.h"
 #include "funcapi.h"
 #include "mb/pg_wchar.h"
@@ -763,6 +764,69 @@ RelationIsVisible(Oid relid)
 	return visible;
 }
 
+/*
+ * VariableIsVisible
+ *		Determine whether a variable (identified by OID) is visible in the
+ *		current search path.  Visible means "would be found by searching
+ *		for the unqualified variable name".
+ */
+bool
+VariableIsVisible(Oid varid)
+{
+	HeapTuple	vartup;
+	Form_pg_variable varform;
+	Oid			varnamespace;
+	bool		visible;
+
+	vartup = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(varid));
+	if (!HeapTupleIsValid(vartup))
+		elog(ERROR, "cache lookup failed for session variable %u", varid);
+	varform = (Form_pg_variable) GETSTRUCT(vartup);
+
+	recomputeNamespacePath();
+
+	/*
+	 * Quick check: if it ain't in the path at all, it ain't visible. Items in
+	 * the system namespace are surely in the path and so we needn't even do
+	 * list_member_oid() for them.
+	 */
+	varnamespace = varform->varnamespace;
+	if (varnamespace != PG_CATALOG_NAMESPACE &&
+		!list_member_oid(activeSearchPath, varnamespace))
+		visible = false;
+	else
+	{
+		/*
+		 * If it is in the path, it might still not be visible; it could be
+		 * hidden by another relation of the same name earlier in the path. So
+		 * we must do a slow check for conflicting relations.
+		 */
+		char	   *varname = NameStr(varform->varname);
+		ListCell   *l;
+
+		visible = false;
+		foreach(l, activeSearchPath)
+		{
+			Oid			namespaceId = lfirst_oid(l);
+
+			if (namespaceId == varnamespace)
+			{
+				/* Found it first in path */
+				visible = true;
+				break;
+			}
+			if (OidIsValid(get_varname_varid(varname, namespaceId)))
+			{
+				/* Found something else first in path */
+				break;
+			}
+		}
+	}
+
+	ReleaseSysCache(vartup);
+
+	return visible;
+}
 
 /*
  * TypenameGetTypid
@@ -2842,6 +2906,302 @@ TSConfigIsVisible(Oid cfgid)
 	return visible;
 }
 
+/*
+ * Returns oid of session variable specified by possibly qualified
+ * identifier.
+ *
+ * If not found, returns InvalidOid if missing_ok, else throws error.
+ */
+Oid
+LookupVariable(const char *nspname, const char *varname, bool missing_ok)
+{
+	Oid			namespaceId;
+	Oid			varoid = InvalidOid;
+	ListCell   *l;
+
+	if (nspname)
+	{
+		namespaceId = LookupExplicitNamespace(nspname, missing_ok);
+
+		/*
+		 * When nspname is not known namespace, then nspname.varname
+		 * cannot to identify any session variable.
+		 */
+		if (!OidIsValid(namespaceId))
+			return InvalidOid;
+
+		varoid = GetSysCacheOid2(VARIABLENAMENSP, Anum_pg_variable_oid,
+								 PointerGetDatum(varname),
+								 ObjectIdGetDatum(namespaceId));
+	}
+	else
+	{
+		/* Iterate over schemas in search_path */
+		recomputeNamespacePath();
+
+		foreach(l, activeSearchPath)
+		{
+			namespaceId = lfirst_oid(l);
+
+			varoid = GetSysCacheOid2(VARIABLENAMENSP, Anum_pg_variable_oid,
+									 PointerGetDatum(varname),
+									 ObjectIdGetDatum(namespaceId));
+
+			if (OidIsValid(varoid))
+				break;
+		}
+	}
+
+	if (!OidIsValid(varoid) && !missing_ok)
+	{
+		if (nspname)
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_OBJECT),
+					 errmsg("session variable \"%s.%s\" does not exist",
+							nspname, varname)));
+		else
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_OBJECT),
+					 errmsg("session variable \"%s\" does not exist",
+							varname)));
+	}
+
+	return varoid;
+}
+
+/*
+ * The input list contains names with indirection expressions used as the left
+ * part of LET statement. The following routine returns a new list with only
+ * initial strings (names) - without indirection expressions.
+ */
+List *
+NamesFromList(List *names)
+{
+	ListCell   *l;
+	List	   *result = NIL;
+
+	foreach(l, names)
+	{
+		Node	   *n = lfirst(l);
+
+		if (IsA(n, String))
+		{
+			result = lappend(result, n);
+		}
+		else
+			break;
+	}
+
+	return result;
+}
+
+/*
+ * IdentifyVariable - try to find variable identified by list of names.
+ *
+ * Before this call we don't know, how these fields should be mapped to
+ * schema name, variable name and attribute name. In this routine
+ * we try to apply passed names to all possible combinations of schema name,
+ * variable name and attribute name, and we count valid combinations.
+ *
+ * When there is not any valid combination, then we are sure, so the
+ * list of names cannot to identify any session variable. In this case
+ * we return InvalidOid.
+ *
+ * We can find more valid combination than one.
+ * Example: users can have session variable x in schema y, and
+ * session variable y with attribute x inside some schema from
+ * search path. In this situation the meaning of expression "y"."x"
+ * is ambiguous. In this case this routine returns InvalidOid, and
+ * sets the output parameter "not_unique" to true. This parameter is
+ * used for more meaningfull error message.
+ *
+ * When lockit is true, then AccessShareLock is created on related
+ * session variable. The lock will be kept for the whole transaction.
+ */
+Oid
+IdentifyVariable(List *names, char **attrname, bool lockit, bool *not_unique)
+{
+	Node	   *field1 = NULL;
+	Node	   *field2 = NULL;
+	Node	   *field3 = NULL;
+	Node	   *field4 = NULL;
+	char	   *a = NULL;
+	char	   *b = NULL;
+	char	   *c = NULL;
+	char	   *d = NULL;
+	Oid			varoid_without_attr = InvalidOid;
+	Oid			varoid_with_attr = InvalidOid;
+	Oid			varid = InvalidOid;
+
+	*not_unique = false;
+
+	switch (list_length(names))
+	{
+		case 1:
+			field1 = linitial(names);
+
+			Assert(IsA(field1, String));
+
+			varid = LookupVariable(NULL, strVal(field1), true);
+			break;
+
+		case 2:
+			field1 = linitial(names);
+			field2 = lsecond(names);
+
+			Assert(IsA(field1, String));
+			a = strVal(field1);
+
+			if (IsA(field2, String))
+			{
+				b = strVal(field2);
+
+				/*
+				 * a.b can mean "schema"."variable" or "variable"."field", Check
+				 * both variants, and returns InvalidOid with not_unique flag, when
+				 * both interpretations are possible. Second node can be star. In
+				 * this case, the only allowed possibility is "variable"."*".
+				 */
+				varoid_without_attr = LookupVariable(a, b, true);
+				varoid_with_attr = LookupVariable(NULL, a, true);
+			}
+			else
+			{
+				Assert(IsA(field2, A_Star));
+
+				/*
+				 * Session variables doesn't support unboxing by star syntax. But
+				 * this syntax have to be calculated here, because can come from
+				 * non session variables related expressions.
+				 */
+				return InvalidOid;
+			}
+
+			if (OidIsValid(varoid_without_attr) && OidIsValid(varoid_with_attr))
+			{
+				*not_unique = true;
+				return InvalidOid;
+			}
+			else if (OidIsValid(varoid_without_attr))
+			{
+				*attrname = NULL;
+				varid = varoid_without_attr;
+			}
+			else
+			{
+				*attrname = b;
+				varid = varoid_with_attr;
+			}
+			break;
+
+		case 3:
+			field1 = linitial(names);
+			field2 = lsecond(names);
+			field3 = lthird(names);
+
+			Assert(IsA(field1, String));
+			Assert(IsA(field2, String));
+
+			a = strVal(field1);
+			b = strVal(field2);
+
+			if (IsA(field3, String))
+			{
+				c = strVal(field3);
+
+				/*
+				 * a.b.c can mean "catalog"."schema"."variable" or
+				 * "schema"."variable"."field", Check both variants, and returns
+				 * InvalidOid with not_unique flag, when both interpretations are
+				 * possible. When third node is star, the only possible
+				 * interpretation is "schema"."variable"."*".
+				 */
+				varoid_without_attr = LookupVariable(b, c, true);
+				varoid_with_attr = LookupVariable(a, b, true);
+			}
+			else
+			{
+				Assert(IsA(field3, A_Star));
+				return InvalidOid;
+			}
+
+			if (OidIsValid(varoid_without_attr) && OidIsValid(varoid_with_attr))
+			{
+				*not_unique = true;
+				return InvalidOid;
+			}
+			else if (OidIsValid(varoid_without_attr))
+			{
+				*attrname = NULL;
+
+				/*
+				 * In this case, "a" is used as catalog name - check it.
+				 */
+				if (strcmp(a, get_database_name(MyDatabaseId)) != 0)
+					ereport(ERROR,
+							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+							 errmsg("cross-database references are not implemented: %s",
+									NameListToString(names))));
+
+				return varoid_without_attr;
+			}
+			else
+			{
+				*attrname = c;
+				return varoid_with_attr;
+			}
+			break;
+
+		case 4:
+			field1 = linitial(names);
+			field2 = lsecond(names);
+			field3 = lthird(names);
+			field4 = lfourth(names);
+
+			Assert(IsA(field1, String));
+			Assert(IsA(field2, String));
+			Assert(IsA(field3, String));
+
+			a = strVal(field1);
+			b = strVal(field2);
+			c = strVal(field3);
+
+			/*
+			 * In this case, "a" is used as catalog name - check it.
+			 */
+			if (strcmp(a, get_database_name(MyDatabaseId)) != 0)
+				ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						 errmsg("cross-database references are not implemented: %s",
+								NameListToString(names))));
+
+			if (IsA(field4, String))
+			{
+				d = strVal(field4);
+			}
+			else
+			{
+				Assert(IsA(field4, A_Star));
+				return InvalidOid;
+			}
+
+			*attrname = d;
+			varid = LookupVariable(b, c, true);
+			break;
+
+		default:
+			ereport(ERROR,
+					(errcode(ERRCODE_SYNTAX_ERROR),
+					 errmsg("improper qualified name (too many dotted names): %s",
+							NameListToString(names))));
+			break;
+	}
+
+	if (OidIsValid(varid) && lockit)
+		LockDatabaseObject(VariableRelationId, varid, 0, AccessShareLock);
+
+	return varid;
+}
 
 /*
  * DeconstructQualifiedName
@@ -4657,3 +5017,14 @@ pg_is_other_temp_schema(PG_FUNCTION_ARGS)
 
 	PG_RETURN_BOOL(isOtherTempNamespace(oid));
 }
+
+Datum
+pg_variable_is_visible(PG_FUNCTION_ARGS)
+{
+	Oid			oid = PG_GETARG_OID(0);
+
+	if (!SearchSysCacheExists1(VARIABLEOID, ObjectIdGetDatum(oid)))
+		PG_RETURN_NULL();
+
+	PG_RETURN_BOOL(VariableIsVisible(oid));
+}
diff --git a/src/backend/catalog/objectaddress.c b/src/backend/catalog/objectaddress.c
index f30c742d48..c2cf06147d 100644
--- a/src/backend/catalog/objectaddress.c
+++ b/src/backend/catalog/objectaddress.c
@@ -62,6 +62,7 @@
 #include "catalog/pg_ts_template.h"
 #include "catalog/pg_type.h"
 #include "catalog/pg_user_mapping.h"
+#include "catalog/pg_variable.h"
 #include "commands/dbcommands.h"
 #include "commands/defrem.h"
 #include "commands/event_trigger.h"
@@ -617,6 +618,20 @@ static const ObjectPropertyType ObjectProperty[] =
 		OBJECT_USER_MAPPING,
 		false
 	},
+	{
+		"session variable",
+		VariableRelationId,
+		VariableObjectIndexId,
+		VARIABLEOID,
+		VARIABLENAMENSP,
+		Anum_pg_variable_oid,
+		Anum_pg_variable_varname,
+		Anum_pg_variable_varnamespace,
+		Anum_pg_variable_varowner,
+		Anum_pg_variable_varacl,
+		OBJECT_VARIABLE,
+		true
+	}
 };
 
 /*
@@ -845,6 +860,10 @@ static const struct object_type_map
 	/* OCLASS_STATISTIC_EXT */
 	{
 		"statistics object", OBJECT_STATISTIC_EXT
+	},
+	/* OCLASS_VARIABLE */
+	{
+		"session variable", OBJECT_VARIABLE
 	}
 };
 
@@ -870,6 +889,7 @@ static ObjectAddress get_object_address_attrdef(ObjectType objtype,
 												bool missing_ok);
 static ObjectAddress get_object_address_type(ObjectType objtype,
 											 TypeName *typename, bool missing_ok);
+static ObjectAddress get_object_address_variable(List *object, bool missing_ok);
 static ObjectAddress get_object_address_opcf(ObjectType objtype, List *object,
 											 bool missing_ok);
 static ObjectAddress get_object_address_opf_member(ObjectType objtype,
@@ -1139,6 +1159,9 @@ get_object_address(ObjectType objtype, Node *object,
 															 missing_ok);
 				address.objectSubId = 0;
 				break;
+			case OBJECT_VARIABLE:
+				address = get_object_address_variable(castNode(List, object), missing_ok);
+				break;
 			default:
 				elog(ERROR, "unrecognized objtype: %d", (int) objtype);
 				/* placate compiler, in case it thinks elog might return */
@@ -2038,16 +2061,20 @@ get_object_address_defacl(List *object, bool missing_ok)
 		case DEFACLOBJ_NAMESPACE:
 			objtype_str = "schemas";
 			break;
+		case DEFACLOBJ_VARIABLE:
+			objtype_str = "variables";
+			break;
 		default:
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 					 errmsg("unrecognized default ACL object type \"%c\"", objtype),
-					 errhint("Valid object types are \"%c\", \"%c\", \"%c\", \"%c\", \"%c\".",
+					 errhint("Valid object types are \"%c\", \"%c\", \"%c\", \"%c\", \"%c\", \"%c\".",
 							 DEFACLOBJ_RELATION,
 							 DEFACLOBJ_SEQUENCE,
 							 DEFACLOBJ_FUNCTION,
 							 DEFACLOBJ_TYPE,
-							 DEFACLOBJ_NAMESPACE)));
+							 DEFACLOBJ_NAMESPACE,
+							 DEFACLOBJ_VARIABLE)));
 	}
 
 	/*
@@ -2132,6 +2159,24 @@ textarray_to_strvaluelist(ArrayType *arr)
 	return list;
 }
 
+/*
+ * Find the ObjectAddress for a session variable
+ */
+static ObjectAddress
+get_object_address_variable(List *object, bool missing_ok)
+{
+	ObjectAddress address;
+	char	   *nspname = NULL;
+	char	   *varname = NULL;
+
+	ObjectAddressSet(address, VariableRelationId, InvalidOid);
+
+	DeconstructQualifiedName(object, &nspname, &varname);
+	address.objectId = LookupVariable(nspname, varname, missing_ok);
+
+	return address;
+}
+
 /*
  * SQL-callable version of get_object_address
  */
@@ -2322,6 +2367,7 @@ pg_get_object_address(PG_FUNCTION_ARGS)
 		case OBJECT_TABCONSTRAINT:
 		case OBJECT_OPCLASS:
 		case OBJECT_OPFAMILY:
+		case OBJECT_VARIABLE:
 			objnode = (Node *) name;
 			break;
 		case OBJECT_ACCESS_METHOD:
@@ -2630,6 +2676,11 @@ check_object_ownership(Oid roleid, ObjectType objtype, ObjectAddress address,
 				aclcheck_error(ACLCHECK_NOT_OWNER, objtype,
 							   NameListToString(castNode(List, object)));
 			break;
+		case OBJECT_VARIABLE:
+			if (!pg_variable_ownercheck(address.objectId, roleid))
+				aclcheck_error(ACLCHECK_NOT_OWNER, objtype,
+							   NameListToString(castNode(List, object)));
+			break;
 		default:
 			elog(ERROR, "unrecognized object type: %d",
 				 (int) objtype);
@@ -3558,6 +3609,32 @@ getObjectDescription(const ObjectAddress *object, bool missing_ok)
 				break;
 			}
 
+		case OCLASS_VARIABLE:
+			{
+				char	   *nspname;
+				HeapTuple	tup;
+				Form_pg_variable varform;
+
+				tup = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(object->objectId));
+				if (!HeapTupleIsValid(tup))
+					elog(ERROR, "cache lookup failed for session variable %u",
+						 object->objectId);
+
+				varform = (Form_pg_variable) GETSTRUCT(tup);
+
+				if (VariableIsVisible(object->objectId))
+					nspname = NULL;
+				else
+					nspname = get_namespace_name(varform->varnamespace);
+
+				appendStringInfo(&buffer, _("session variable %s"),
+								 quote_qualified_identifier(nspname,
+															NameStr(varform->varname)));
+
+				ReleaseSysCache(tup);
+				break;
+			}
+
 		case OCLASS_TSPARSER:
 			{
 				HeapTuple	tup;
@@ -3868,6 +3945,16 @@ getObjectDescription(const ObjectAddress *object, bool missing_ok)
 										 _("default privileges on new schemas belonging to role %s"),
 										 rolename);
 						break;
+					case DEFACLOBJ_VARIABLE:
+						if (nspname)
+							appendStringInfo(&buffer,
+											 _("default privileges on new session variables belonging to role %s in schema %s"),
+											 rolename, nspname);
+						else
+							appendStringInfo(&buffer,
+											 _("default privileges on new session variables belonging to role %s"),
+											 rolename);
+						break;
 					default:
 						/* shouldn't get here */
 						if (nspname)
@@ -4610,6 +4697,10 @@ getObjectTypeDescription(const ObjectAddress *object, bool missing_ok)
 			appendStringInfoString(&buffer, "transform");
 			break;
 
+		case OCLASS_VARIABLE:
+			appendStringInfoString(&buffer, "session variable");
+			break;
+
 			/*
 			 * There's intentionally no default: case here; we want the
 			 * compiler to warn if a new OCLASS hasn't been handled above.
@@ -5703,6 +5794,10 @@ getObjectIdentityParts(const ObjectAddress *object,
 						appendStringInfoString(&buffer,
 											   " on schemas");
 						break;
+					case DEFACLOBJ_VARIABLE:
+						appendStringInfoString(&buffer,
+											   " on session variables");
+						break;
 				}
 
 				if (objname)
@@ -5918,6 +6013,33 @@ getObjectIdentityParts(const ObjectAddress *object,
 			}
 			break;
 
+		case OCLASS_VARIABLE:
+			{
+				char	   *schema;
+				char	   *varname;
+				HeapTuple	tup;
+				Form_pg_variable varform;
+
+				tup = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(object->objectId));
+				if (!HeapTupleIsValid(tup))
+					elog(ERROR, "cache lookup failed for session variable %u",
+						 object->objectId);
+
+				varform = (Form_pg_variable) GETSTRUCT(tup);
+
+				schema = get_namespace_name_or_temp(varform->varnamespace);
+				varname = NameStr(varform->varname);
+
+				appendStringInfo(&buffer, "%s",
+								 quote_qualified_identifier(schema, varname));
+
+				if (objname)
+					*objname = list_make2(schema, varname);
+
+				ReleaseSysCache(tup);
+				break;
+			}
+
 			/*
 			 * There's intentionally no default: case here; we want the
 			 * compiler to warn if a new OCLASS hasn't been handled above.
diff --git a/src/backend/catalog/pg_shdepend.c b/src/backend/catalog/pg_shdepend.c
index 3e8fa008b9..cb58d9a0fd 100644
--- a/src/backend/catalog/pg_shdepend.c
+++ b/src/backend/catalog/pg_shdepend.c
@@ -46,6 +46,7 @@
 #include "catalog/pg_ts_dict.h"
 #include "catalog/pg_type.h"
 #include "catalog/pg_user_mapping.h"
+#include "catalog/pg_variable.h"
 #include "commands/alter.h"
 #include "commands/collationcmds.h"
 #include "commands/conversioncmds.h"
@@ -1593,6 +1594,7 @@ shdepReassignOwned(List *roleids, Oid newrole)
 				case DatabaseRelationId:
 				case TSConfigRelationId:
 				case TSDictionaryRelationId:
+				case VariableRelationId:
 					{
 						Oid			classId = sdepForm->classid;
 						Relation	catalog;
diff --git a/src/backend/catalog/pg_variable.c b/src/backend/catalog/pg_variable.c
new file mode 100644
index 0000000000..fa81b54f59
--- /dev/null
+++ b/src/backend/catalog/pg_variable.c
@@ -0,0 +1,396 @@
+/*-------------------------------------------------------------------------
+ *
+ * pg_variable.c
+ *		session variables
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/backend/catalog/pg_variable.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "access/heapam.h"
+#include "access/htup_details.h"
+
+#include "catalog/catalog.h"
+#include "catalog/dependency.h"
+#include "catalog/indexing.h"
+#include "catalog/namespace.h"
+#include "catalog/objectaccess.h"
+#include "catalog/pg_collation.h"
+#include "catalog/pg_namespace.h"
+#include "catalog/pg_type.h"
+#include "catalog/pg_variable.h"
+#include "commands/session_variable.h"
+#include "storage/lmgr.h"
+#include "utils/builtins.h"
+#include "utils/lsyscache.h"
+#include "utils/syscache.h"
+
+static VariableEOXActionCodes
+to_eoxaction_code(VariableEOXAction action)
+{
+	switch (action)
+	{
+		case VARIABLE_EOX_NOOP:
+			return VARIABLE_EOX_CODE_NOOP;
+
+		case VARIABLE_EOX_DROP:
+			return VARIABLE_EOX_CODE_DROP;
+
+		case VARIABLE_EOX_RESET:
+			return VARIABLE_EOX_CODE_RESET;
+
+		default:
+			elog(ERROR, "unexpected action");
+	}
+}
+
+static VariableEOXAction
+to_eoxaction(VariableEOXActionCodes code)
+{
+	switch (code)
+	{
+		case VARIABLE_EOX_CODE_NOOP:
+			return VARIABLE_EOX_NOOP;
+
+		case VARIABLE_EOX_CODE_DROP:
+			return VARIABLE_EOX_DROP;
+
+		case VARIABLE_EOX_CODE_RESET:
+			return VARIABLE_EOX_RESET;
+
+		default:
+			elog(ERROR, "unexpected code");
+	}
+}
+
+/*
+ * Returns name of session variable. When the variable is not in the path,
+ * the name is qualified.
+ */
+char *
+session_variable_get_name(Oid varid)
+{
+	HeapTuple	tup;
+	Form_pg_variable varform;
+	char	   *varname;
+	char	   *nspname;
+	char	   *result;
+
+	tup = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(varid));
+
+	if (!HeapTupleIsValid(tup))
+		elog(ERROR, "cache lookup failed for variable %u", varid);
+
+	varform = (Form_pg_variable) GETSTRUCT(tup);
+
+	varname = NameStr(varform->varname);
+
+	if (VariableIsVisible(varid))
+		nspname = NULL;
+	else
+		nspname = get_namespace_name(varform->varnamespace);
+
+	result = quote_qualified_identifier(nspname, varname);
+
+	ReleaseSysCache(tup);
+
+	return result;
+}
+
+/*
+ * Returns varname field of pg_variable
+ */
+char *
+get_session_variable_name(Oid varid)
+{
+	HeapTuple	tup;
+	Form_pg_variable varform;
+	char	   *varname;
+
+	tup = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(varid));
+
+	if (!HeapTupleIsValid(tup))
+		elog(ERROR, "cache lookup failed for session variable %u", varid);
+
+	varform = (Form_pg_variable) GETSTRUCT(tup);
+
+	varname = NameStr(varform->varname);
+
+	ReleaseSysCache(tup);
+
+	return varname;
+}
+
+/*
+ * Returns type, typmod and collid of session variable.
+ */
+void
+get_session_variable_type_typmod_collid(Oid varid, Oid *typid, int32 *typmod,
+										Oid *collid)
+{
+	HeapTuple	tup;
+	Form_pg_variable varform;
+
+	tup = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(varid));
+
+	if (!HeapTupleIsValid(tup))
+		elog(ERROR, "cache lookup failed for session variable %u", varid);
+
+	varform = (Form_pg_variable) GETSTRUCT(tup);
+
+	*typid = varform->vartype;
+	*typmod = varform->vartypmod;
+	*collid = varform->varcollation;
+
+	ReleaseSysCache(tup);
+
+	return;
+}
+
+/*
+ * Fetch all fields of session variable from the syscache.
+ */
+void
+initVariable(Variable *var, Oid varid, bool missing_ok, bool fast_only)
+{
+	HeapTuple	tup;
+	Form_pg_variable varform;
+	Datum		defexpr_datum;
+	bool		defexpr_isnull;
+
+	tup = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(varid));
+
+	if (!HeapTupleIsValid(tup))
+	{
+		if (missing_ok)
+		{
+			var->oid = InvalidOid;
+			return;
+		}
+
+		elog(ERROR, "cache lookup failed for session variable %u", varid);
+	}
+
+	varform = (Form_pg_variable) GETSTRUCT(tup);
+
+	var->oid = varid;
+	var->name = pstrdup(NameStr(varform->varname));
+	var->namespace = varform->varnamespace;
+	var->typid = varform->vartype;
+	var->typmod = varform->vartypmod;
+	var->owner = varform->varowner;
+	var->collation = varform->varcollation;
+	var->eoxaction = to_eoxaction(varform->vareoxaction);
+	var->is_not_null = varform->varisnotnull;
+	var->is_immutable = varform->varisimmutable;
+
+	/* Get defexpr */
+	defexpr_datum = SysCacheGetAttr(VARIABLEOID,
+									tup,
+									Anum_pg_variable_vardefexpr,
+									&defexpr_isnull);
+
+	var->has_defexpr = !defexpr_isnull;
+
+	/*
+	 * Sometimes we don't need to deserialize defexpr, but we need info about
+	 * the existence of defexpr.
+	 */
+	if (!fast_only)
+	{
+		Datum		aclDatum;
+		bool		isnull;
+
+		/* name */
+		var->name = pstrdup(NameStr(varform->varname));
+
+		if (!defexpr_isnull)
+			var->defexpr = stringToNode(TextDatumGetCString(defexpr_datum));
+		else
+			var->defexpr = NULL;
+
+		/* Get varacl */
+		aclDatum = SysCacheGetAttr(VARIABLEOID,
+								   tup,
+								   Anum_pg_variable_varacl,
+								   &isnull);
+		if (!isnull)
+			var->acl = DatumGetAclPCopy(aclDatum);
+		else
+			var->acl = NULL;
+	}
+	else
+	{
+		var->name = NULL;
+		var->defexpr = NULL;
+		var->acl = NULL;
+	}
+
+	ReleaseSysCache(tup);
+
+	return;
+}
+
+/*
+ * Create entry in pg_variable table
+ */
+ObjectAddress
+VariableCreate(const char *varName,
+			   Oid varNamespace,
+			   Oid varType,
+			   int32 varTypmod,
+			   Oid varOwner,
+			   Oid varCollation,
+			   Node *varDefexpr,
+			   VariableEOXAction eoxaction,
+			   bool is_not_null,
+			   bool if_not_exists,
+			   bool is_immutable)
+{
+	Acl		   *varacl;
+	NameData	varname;
+	bool		nulls[Natts_pg_variable];
+	Datum		values[Natts_pg_variable];
+	Relation	rel;
+	HeapTuple	tup,
+				oldtup;
+	TupleDesc	tupdesc;
+	ObjectAddress myself,
+				referenced;
+	Oid			varid;
+	int			i;
+
+	for (i = 0; i < Natts_pg_variable; i++)
+	{
+		nulls[i] = false;
+		values[i] = (Datum) 0;
+	}
+
+	namestrcpy(&varname, varName);
+
+	rel = table_open(VariableRelationId, RowExclusiveLock);
+
+	varid = GetNewOidWithIndex(rel, VariableObjectIndexId, Anum_pg_variable_oid);
+	values[Anum_pg_variable_oid - 1] = ObjectIdGetDatum(varid);
+	values[Anum_pg_variable_varname - 1] = NameGetDatum(&varname);
+	values[Anum_pg_variable_varnamespace - 1] = ObjectIdGetDatum(varNamespace);
+	values[Anum_pg_variable_vartype - 1] = ObjectIdGetDatum(varType);
+	values[Anum_pg_variable_vartypmod - 1] = Int32GetDatum(varTypmod);
+	values[Anum_pg_variable_varowner - 1] = ObjectIdGetDatum(varOwner);
+	values[Anum_pg_variable_varcollation - 1] = ObjectIdGetDatum(varCollation);
+	values[Anum_pg_variable_vareoxaction - 1] = CharGetDatum((char) to_eoxaction_code(eoxaction));
+	values[Anum_pg_variable_varisnotnull - 1] = BoolGetDatum(is_not_null);
+	values[Anum_pg_variable_varisimmutable - 1] = BoolGetDatum(is_immutable);
+	/* varacl will be determined later */
+
+	if (varDefexpr)
+		values[Anum_pg_variable_vardefexpr - 1] = CStringGetTextDatum(nodeToString(varDefexpr));
+	else
+		nulls[Anum_pg_variable_vardefexpr - 1] = true;
+
+	tupdesc = RelationGetDescr(rel);
+
+	oldtup = SearchSysCache2(VARIABLENAMENSP,
+							 PointerGetDatum(varName),
+							 ObjectIdGetDatum(varNamespace));
+
+	if (HeapTupleIsValid(oldtup))
+	{
+		if (if_not_exists)
+			ereport(NOTICE,
+					(errcode(ERRCODE_DUPLICATE_OBJECT),
+					 errmsg("session variable \"%s\" already exists, skipping",
+							varName)));
+		else
+			ereport(ERROR,
+					(errcode(ERRCODE_DUPLICATE_OBJECT),
+					 errmsg("session variable \"%s\" already exists",
+							varName)));
+
+		ReleaseSysCache(oldtup);
+		table_close(rel, RowExclusiveLock);
+
+		return InvalidObjectAddress;
+	}
+
+	varacl = get_user_default_acl(OBJECT_VARIABLE, varOwner,
+								  varNamespace);
+
+	if (varacl != NULL)
+		values[Anum_pg_variable_varacl - 1] = PointerGetDatum(varacl);
+	else
+		nulls[Anum_pg_variable_varacl - 1] = true;
+
+	tup = heap_form_tuple(tupdesc, values, nulls);
+	CatalogTupleInsert(rel, tup);
+
+	myself.classId = VariableRelationId;
+	myself.objectId = varid;
+	myself.objectSubId = 0;
+
+	/* dependency on namespace */
+	referenced.classId = NamespaceRelationId;
+	referenced.objectId = varNamespace;
+	referenced.objectSubId = 0;
+	recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL);
+
+	/* dependency on used type */
+	referenced.classId = TypeRelationId;
+	referenced.objectId = varType;
+	referenced.objectSubId = 0;
+	recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL);
+
+	/* dependency on default expr */
+	if (varDefexpr)
+		recordDependencyOnExpr(&myself, (Node *) varDefexpr,
+							   NIL, DEPENDENCY_NORMAL);
+
+	/* dependency on any roles mentioned in ACL */
+	if (varacl != NULL)
+	{
+		int			nnewmembers;
+		Oid		   *newmembers;
+
+		nnewmembers = aclmembers(varacl, &newmembers);
+		updateAclDependencies(VariableRelationId, varid, 0,
+							  varOwner,
+							  0, NULL,
+							  nnewmembers, newmembers);
+	}
+
+	/* dependency on extension */
+	recordDependencyOnCurrentExtension(&myself, false);
+
+	/* dependency on collation */
+	if (OidIsValid(varCollation) &&
+		varCollation != DEFAULT_COLLATION_OID)
+	{
+		referenced.classId = CollationRelationId;
+		referenced.objectId = varCollation;
+		referenced.objectSubId = 0;
+		recordDependencyOn(&myself, &referenced, DEPENDENCY_NORMAL);
+	}
+
+	/*
+	 * When this new field in catalog is temporal, we need to create
+	 * new xact action to ensure delete from catalog.
+	 */
+	if (eoxaction == VARIABLE_EOX_DROP)
+		RegisterOnCommitDropSessionVariable(myself.objectId);
+
+	heap_freetuple(tup);
+
+	/* Post creation hook for new function */
+	InvokeObjectPostCreateHook(VariableRelationId, varid, 0);
+
+	table_close(rel, RowExclusiveLock);
+
+	return myself;
+}
diff --git a/src/backend/commands/Makefile b/src/backend/commands/Makefile
index 48f7348f91..41f98f8ba0 100644
--- a/src/backend/commands/Makefile
+++ b/src/backend/commands/Makefile
@@ -48,6 +48,7 @@ OBJS = \
 	proclang.o \
 	publicationcmds.o \
 	schemacmds.o \
+	sessionvariable.o \
 	seclabel.o \
 	sequence.o \
 	statscmds.o \
diff --git a/src/backend/commands/alter.c b/src/backend/commands/alter.c
index 1f64c8aa51..eb6c1c0b7f 100644
--- a/src/backend/commands/alter.c
+++ b/src/backend/commands/alter.c
@@ -40,6 +40,7 @@
 #include "catalog/pg_ts_dict.h"
 #include "catalog/pg_ts_parser.h"
 #include "catalog/pg_ts_template.h"
+#include "catalog/pg_variable.h"
 #include "commands/alter.h"
 #include "commands/collationcmds.h"
 #include "commands/conversioncmds.h"
@@ -141,6 +142,10 @@ report_namespace_conflict(Oid classId, const char *name, Oid nspOid)
 			Assert(OidIsValid(nspOid));
 			msgfmt = gettext_noop("text search configuration \"%s\" already exists in schema \"%s\"");
 			break;
+		case VariableRelationId:
+			Assert(OidIsValid(nspOid));
+			msgfmt = gettext_noop("session variable \"%s\" already exists in schema \"%s\"");
+			break;
 		default:
 			elog(ERROR, "unsupported object class %u", classId);
 			break;
@@ -393,6 +398,7 @@ ExecRenameStmt(RenameStmt *stmt)
 		case OBJECT_TSTEMPLATE:
 		case OBJECT_PUBLICATION:
 		case OBJECT_SUBSCRIPTION:
+		case OBJECT_VARIABLE:
 			{
 				ObjectAddress address;
 				Relation	catalog;
@@ -536,6 +542,7 @@ ExecAlterObjectSchemaStmt(AlterObjectSchemaStmt *stmt,
 		case OBJECT_TSDICTIONARY:
 		case OBJECT_TSPARSER:
 		case OBJECT_TSTEMPLATE:
+		case OBJECT_VARIABLE:
 			{
 				Relation	catalog;
 				Relation	relation;
@@ -626,6 +633,7 @@ AlterObjectNamespace_oid(Oid classId, Oid objid, Oid nspOid,
 		case OCLASS_TSDICT:
 		case OCLASS_TSTEMPLATE:
 		case OCLASS_TSCONFIG:
+		case OCLASS_VARIABLE:
 			{
 				Relation	catalog;
 
@@ -884,6 +892,7 @@ ExecAlterOwnerStmt(AlterOwnerStmt *stmt)
 		case OBJECT_TABLESPACE:
 		case OBJECT_TSDICTIONARY:
 		case OBJECT_TSCONFIGURATION:
+		case OBJECT_VARIABLE:
 			{
 				Relation	catalog;
 				Relation	relation;
diff --git a/src/backend/commands/discard.c b/src/backend/commands/discard.c
index c583539e0c..68fe550a71 100644
--- a/src/backend/commands/discard.c
+++ b/src/backend/commands/discard.c
@@ -19,6 +19,7 @@
 #include "commands/discard.h"
 #include "commands/prepare.h"
 #include "commands/sequence.h"
+#include "commands/session_variable.h"
 #include "utils/guc.h"
 #include "utils/portal.h"
 
@@ -48,6 +49,10 @@ DiscardCommand(DiscardStmt *stmt, bool isTopLevel)
 			ResetTempTableNamespace();
 			break;
 
+		case DISCARD_VARIABLES:
+			ResetSessionVariables();
+			break;
+
 		default:
 			elog(ERROR, "unrecognized DISCARD target: %d", stmt->target);
 	}
@@ -75,4 +80,5 @@ DiscardAll(bool isTopLevel)
 	ResetPlanCache();
 	ResetTempTableNamespace();
 	ResetSequenceCaches();
+	ResetSessionVariables();
 }
diff --git a/src/backend/commands/dropcmds.c b/src/backend/commands/dropcmds.c
index c9b5732448..e526143bdc 100644
--- a/src/backend/commands/dropcmds.c
+++ b/src/backend/commands/dropcmds.c
@@ -481,6 +481,10 @@ does_not_exist_skipping(ObjectType objtype, Node *object)
 			msg = gettext_noop("publication \"%s\" does not exist, skipping");
 			name = strVal(object);
 			break;
+		case OBJECT_VARIABLE:
+			msg = gettext_noop("session variable \"%s\" does not exist, skipping");
+			name = NameListToString(castNode(List, object));
+			break;
 		default:
 			elog(ERROR, "unrecognized object type: %d", (int) objtype);
 			break;
diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c
index 93c2099735..2766fa6293 100644
--- a/src/backend/commands/event_trigger.c
+++ b/src/backend/commands/event_trigger.c
@@ -991,6 +991,7 @@ EventTriggerSupportsObjectType(ObjectType obtype)
 		case OBJECT_TSTEMPLATE:
 		case OBJECT_TYPE:
 		case OBJECT_USER_MAPPING:
+		case OBJECT_VARIABLE:
 		case OBJECT_VIEW:
 			return true;
 
@@ -1055,6 +1056,7 @@ EventTriggerSupportsObjectClass(ObjectClass objclass)
 		case OCLASS_PUBLICATION_REL:
 		case OCLASS_SUBSCRIPTION:
 		case OCLASS_TRANSFORM:
+		case OCLASS_VARIABLE:
 			return true;
 
 			/*
@@ -2106,6 +2108,8 @@ stringify_grant_objtype(ObjectType objtype)
 			return "TABLESPACE";
 		case OBJECT_TYPE:
 			return "TYPE";
+		case OBJECT_VARIABLE:
+			return "VARIABLE";
 			/* these currently aren't used */
 		case OBJECT_ACCESS_METHOD:
 		case OBJECT_AGGREGATE:
@@ -2189,6 +2193,8 @@ stringify_adefprivs_objtype(ObjectType objtype)
 			return "TABLESPACES";
 		case OBJECT_TYPE:
 			return "TYPES";
+		case OBJECT_VARIABLE:
+			return "VARIABLES";
 			/* these currently aren't used */
 		case OBJECT_ACCESS_METHOD:
 		case OBJECT_AGGREGATE:
diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index b970997c34..05bf42d53e 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -492,6 +492,22 @@ ExplainOneUtility(Node *utilityStmt, IntoClause *into, ExplainState *es,
 		else
 			ExplainDummyGroup("Notify", NULL, es);
 	}
+	else if (IsA(utilityStmt, LetStmt))
+	{
+		LetStmt	   *letstmt = (LetStmt *) utilityStmt;
+		List	   *rewritten;
+
+		if (es->format == EXPLAIN_FORMAT_TEXT)
+			appendStringInfoString(es->str, "SET SESSION VARIABLE\n");
+		else
+			ExplainDummyGroup("Set Session Variable", NULL, es);
+
+		rewritten = QueryRewrite(castNode(Query, copyObject(letstmt->query)));
+		Assert(list_length(rewritten) == 1);
+		ExplainOneQuery(linitial_node(Query, rewritten),
+						CURSOR_OPT_PARALLEL_OK, NULL, es,
+						queryString, params, queryEnv);
+	}
 	else
 	{
 		if (es->format == EXPLAIN_FORMAT_TEXT)
diff --git a/src/backend/commands/seclabel.c b/src/backend/commands/seclabel.c
index 7a62d547e2..a731431563 100644
--- a/src/backend/commands/seclabel.c
+++ b/src/backend/commands/seclabel.c
@@ -91,6 +91,7 @@ SecLabelSupportsObjectType(ObjectType objtype)
 		case OBJECT_TSPARSER:
 		case OBJECT_TSTEMPLATE:
 		case OBJECT_USER_MAPPING:
+		case OBJECT_VARIABLE:
 			return false;
 
 			/*
diff --git a/src/backend/commands/sessionvariable.c b/src/backend/commands/sessionvariable.c
new file mode 100644
index 0000000000..0e058b693e
--- /dev/null
+++ b/src/backend/commands/sessionvariable.c
@@ -0,0 +1,1109 @@
+/*-------------------------------------------------------------------------
+ *
+ * sessionvariable.c
+ *	  session variable creation/manipulation commands
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/commands/sessionvariable.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+#include "miscadmin.h"
+#include "access/heapam.h"
+#include "access/htup_details.h"
+#include "access/xact.h"
+#include "catalog/dependency.h"
+#include "catalog/indexing.h"
+#include "catalog/namespace.h"
+#include "catalog/pg_variable.h"
+#include "commands/session_variable.h"
+#include "executor/executor.h"
+#include "executor/svariableReceiver.h"
+#include "optimizer/optimizer.h"
+#include "parser/parse_coerce.h"
+#include "parser/parse_collate.h"
+#include "parser/parse_expr.h"
+#include "parser/parse_type.h"
+#include "rewrite/rewriteHandler.h"
+#include "storage/lmgr.h"
+#include "tcop/tcopprot.h"
+#include "utils/builtins.h"
+#include "utils/datum.h"
+#include "utils/inval.h"
+#include "utils/lsyscache.h"
+#include "utils/snapmgr.h"
+#include "utils/syscache.h"
+
+/*
+ * An values of session variables are stored in local memory
+ * in sessionvars hash table. This local memory have to be
+ * cleaned, when a) session variable is dropped by current or
+ * by other session, b) when user enforce it by using clause
+ * ON TRANSACTION END RESET. The life cycle of temporal session
+ * variable can be limmited by using clause ON COMMIT DROP.
+ *
+ * Although session variables are not transactional, we don't
+ * want (and we cannot) to run cleaning immediately (when we
+ * got sinval message). The value of session variables can
+ * be still used or the operation that emits cleaning can be
+ * reverted. Unfortunatelly, this check can be done only in
+ * when transaction is committed (the check against system
+ * catalog requires transaction state).
+ *
+ * This is reason why cleaning memory (session variable reset)
+ * is postponed to end of transaction, and we need to hold
+ * some actions lists. We have to hold two separate action
+ * lists - one for dropping from system catalog, and second
+ * for resetting. It's necessary, because dropping session
+ * variable enforces session variable reset. The drop operation
+ * can be executed when we iterate over action list, and in
+ * this moment we would not to modify same action list.
+ *
+ * We want to support possibility to reset session variable at
+ * the end of transaction. This ensure initial state of session
+ * variable at the begin of each transaction. The reset is
+ * implemented like removing variable from sessionvars hash table.
+ * This enforce full initialization in next usage.
+ * Attention - this is not same as drop session variable.
+ *
+ * Another functionality is dropping temporary session variable
+ * with the option ON COMMIT DROP.
+ */
+typedef enum SVariableXActAction
+{
+	ON_COMMIT_DROP,		/* used for ON COMMIT DROP */
+	ON_COMMIT_RESET,	/* used for DROP VARIABLE */
+	RESET,				/* used for ON TRANSACTION END RESET */
+	RECHECK				/* recheck if session variable is living */
+} SVariableXActAction;
+
+typedef struct SVariableXActActionItem
+{
+	Oid			varid;			/* varid of session variable */
+	SVariableXActAction action;	/* reset or drop */
+
+	/*
+	 * If this entry was created during the current transaction,
+	 * creating_subid is the ID of the creating subxact; if created in a prior
+	 * transaction, creating_subid is zero.  If deleted during the current
+	 * transaction, deleting_subid is the ID of the deleting subxact; if no
+	 * deletion request is pending, deleting_subid is zero.
+	 */
+	SubTransactionId creating_subid;
+	SubTransactionId deleting_subid;
+}  SVariableXActActionItem;
+
+static List *xact_drop_actions = NIL;
+static List *xact_reset_actions = NIL;
+
+typedef struct SVariableData
+{
+	Oid			varid;			/* pg_variable OID of this sequence (hash key) */
+	Oid			typid;			/* OID of the data type */
+	int16		typlen;
+	bool		typbyval;
+	bool		isnull;
+	bool		freeval;
+	Datum		value;
+
+	bool		is_rowtype;		/* true when variable is composite */
+	bool		is_not_null;	/* don't allow null values */
+	bool		is_immutable;	/* true when variable is immutable */
+	bool		has_defexpr;	/* true when there are default value */
+
+	bool		is_valid;		/* true when variable was successfuly
+								 * initialized */
+
+	uint32		hashvalue;
+}			SVariableData;
+
+typedef SVariableData * SVariable;
+
+static HTAB *sessionvars = NULL;	/* hash table for session variables */
+static MemoryContext SVariableMemoryContext = NULL;
+
+static bool first_time = true;
+
+static bool sync_sessionvars_all = false;
+
+static void register_session_variable_xact_action(Oid varid, SVariableXActAction action);
+static void delete_session_variable_xact_action(Oid varid, SVariableXActAction action);
+
+/*
+ * Releases stored data from session variable. The hash entry
+ * stay without change.
+ */
+static void
+free_session_variable_value(SVariable svar)
+{
+	if (svar->freeval)
+		pfree(DatumGetPointer(svar->value));
+
+	/* Clean current value */
+	svar->value = (Datum) 0;
+	svar->isnull = true;
+	svar->freeval = false;
+
+	/*
+	 * We can mark this session variable as valid when
+	 * it has not default expression, and when null is
+	 * allowed. When it has defexpr, then the content
+	 * will be valid after an assignment or defexp evaluation.
+	 */
+	svar->is_valid = !svar->has_defexpr && !svar->is_not_null;
+}
+
+/*
+ * Release the variable defined by varid from sessionvars
+ * hashtab.
+ */
+static void
+free_session_variable(SVariable svar)
+{
+	free_session_variable_value(svar);
+
+	if (hash_search(sessionvars,
+					(void *) &svar->varid,
+					HASH_REMOVE,
+					NULL) == NULL)
+		elog(DEBUG1, "hash table corrupted");
+}
+
+/*
+ * Release the variable defined by varid from sessionvars
+ * hashtab.
+ */
+static void
+free_session_variable_varid(Oid varid)
+{
+	SVariable svar;
+	bool		found;
+
+	if (!sessionvars)
+		return;
+
+	svar = (SVariable) hash_search(sessionvars, &varid,
+										HASH_FIND, &found);
+	if (found)
+		free_session_variable(svar);
+}
+
+/*
+ * Assign sinval mark to session variable. This mark probably
+ * signalized, so the session variable was dropped. But this
+ * should be rechecked later against system catalog.
+ */
+static void
+pg_variable_cache_callback(Datum arg, int cacheid, uint32 hashvalue)
+{
+	/*
+	 * We can have not sessionvars in initialized state, and
+	 * we can get this message. This is possible after execution of
+	 * DISCARD VARIABLES command.
+	 */
+	if (!sessionvars)
+		return;
+
+	/*
+	 * When we know so all session variables should be synchronized,
+	 * then is useless to continue;
+	 */
+	if (sync_sessionvars_all)
+		return;
+
+	/*
+	 * Because we cannot to decode varid from hashValue, we should
+	 * to iterate over all currently used session variables to find
+	 * session variable with same hashValue. On second hand, this can
+	 * save us some CPU later, because we don't need to check any used
+	 * session variable (by current session) against system catalog.
+	 */
+	if (hashvalue != 0)
+	{
+		HASH_SEQ_STATUS status;
+		SVariable svar;
+
+		hash_seq_init(&status, sessionvars);
+
+		while ((svar = (SVariable) hash_seq_search(&status)) != NULL)
+		{
+			if (svar->hashvalue == hashvalue)
+				register_session_variable_xact_action(svar->varid, RECHECK);
+
+			/*
+			 * although it there is low probability, we have to iterate
+			 * over all actively used session variables, because hashvalue
+			 * is not unique identifier.
+			 */
+		}
+	}
+	else
+		sync_sessionvars_all = true;
+}
+
+/*
+ * When we need to recheck all session variables, then
+ * the most effective method is seq scan over hash tab.
+ */
+static void
+sync_sessionvars_xact_callback(XactEvent event, void *arg)
+{
+	HASH_SEQ_STATUS status;
+	SVariable svar;
+
+	if (!sync_sessionvars_all)
+		return;
+
+	/*
+	 * sessionvars is null after DISCARD VARIABLES.
+	 * When we are sure, so there are not any
+	 * active session variable in this session, we
+	 * can reset sync_sessionvars_all flag.
+	 */
+	if (!sessionvars)
+	{
+		sync_sessionvars_all = false;
+		return;
+	}
+
+	/*
+	 * This callback can be called when database is not in
+	 * transaction state - XACT_EVENT_ABORT, XACT_EVENT_COMMIT
+	 * but the checks against system catalog needs transaction state.
+	 */
+	if (!IsTransactionState())
+		return;
+
+	hash_seq_init(&status, sessionvars);
+
+	while ((svar = (SVariable) hash_seq_search(&status)) != NULL)
+	{
+		HeapTuple	tp;
+
+		tp = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(svar->varid));
+
+		if (HeapTupleIsValid(tp))
+			ReleaseSysCache(tp);
+		else
+			free_session_variable(svar);
+	}
+
+	sync_sessionvars_all = false;
+}
+
+/*
+ * Create the hash table for storing session variables
+ */
+static void
+create_sessionvars_hashtable(void)
+{
+	HASHCTL		ctl;
+
+	/* set callbacks */
+	if (first_time)
+	{
+		/* Read sinval messages */
+		CacheRegisterSyscacheCallback(VARIABLEOID,
+									  pg_variable_cache_callback,
+									  (Datum) 0);
+
+		/* at transaction end recheck sinvalidated variables */
+		RegisterXactCallback(sync_sessionvars_xact_callback, NULL);
+
+		first_time = false;
+	}
+
+	/* needs its own long lived memory context */
+	if (SVariableMemoryContext == NULL)
+	{
+		SVariableMemoryContext =
+			AllocSetContextCreate(TopMemoryContext,
+								  "session variables",
+								  ALLOCSET_START_SMALL_SIZES);
+	}
+
+	memset(&ctl, 0, sizeof(ctl));
+	ctl.keysize = sizeof(Oid);
+	ctl.entrysize = sizeof(SVariableData);
+	ctl.hcxt = SVariableMemoryContext;
+
+	Assert(sessionvars == NULL);
+
+	sessionvars = hash_create("Session variables", 64, &ctl,
+								   HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+}
+
+/*
+ * Assign some content to the session variable. It's copied to
+ * SVariableMemoryContext if it is necessary.
+ *
+ * init_mode is true, when the value of session variable is initialized
+ * by default expression or by null. Only in this moment we can allow to
+ * modify immutable variables with default expression.
+ */
+static void
+set_session_variable(SVariable svar, Datum value,
+					 bool isnull, Oid typid,
+					 bool init_mode)
+{
+	MemoryContext oldcxt;
+	Datum		newval = value;
+
+	/* Don't allow assignment of null to NOT NULL variable */
+	if (isnull && svar->is_not_null)
+		ereport(ERROR,
+				(errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
+				 errmsg("null value is not allowed for NOT NULL session variable \"%s\"",
+						session_variable_get_name(svar->varid))));
+
+	if (!isnull && svar->typid != typid)
+		ereport(ERROR,
+				(errcode(ERRCODE_DATATYPE_MISMATCH),
+				 errmsg("type \"%s\" of assigned value is different than type \"%s\" of session variable \"%s\"",
+						format_type_be(typid),
+						format_type_be(svar->typid),
+						session_variable_get_name(svar->varid))));
+
+	/*
+	 * Don't allow updating of immutable session variable that has assigned
+	 * not null value or has default expression (and then the value should be
+	 * result of default expression always). Don't do this check, when variable
+	 * is initialized.
+	 */
+	if (!init_mode &&
+		(svar->is_immutable && (!svar->isnull || svar->has_defexpr)))
+		ereport(ERROR,
+				(errcode(ERRCODE_ERROR_IN_ASSIGNMENT),
+				 errmsg("session variable \"%s\" is declared IMMUTABLE",
+						session_variable_get_name(svar->varid))));
+
+	/* copy value to session persistent context */
+	oldcxt = MemoryContextSwitchTo(SVariableMemoryContext);
+	if (!isnull)
+		newval = datumCopy(value, svar->typbyval, svar->typlen);
+	MemoryContextSwitchTo(oldcxt);
+
+	free_session_variable_value(svar);
+
+	svar->value = newval;
+	svar->isnull = isnull;
+	svar->freeval = newval != value;
+	svar->is_valid = true;
+}
+
+/*
+ * Initialize svar from var
+ * svar - SVariable - holds data
+ * var  - Variable - holds metadata
+ */
+static void
+init_session_variable(SVariable svar, Variable *var)
+{
+	Assert(OidIsValid(var->oid));
+
+	svar->varid = var->oid;
+	svar->typid = var->typid;
+
+	get_typlenbyval(var->typid,
+					&svar->typlen,
+					&svar->typbyval);
+
+	svar->isnull = true;
+	svar->freeval = false;
+	svar->value = (Datum) 0;
+
+	svar->is_rowtype = type_is_rowtype(var->typid);
+	svar->is_not_null = var->is_not_null;
+	svar->is_immutable = var->is_immutable;
+	svar->has_defexpr = var->has_defexpr;
+
+	svar->hashvalue = GetSysCacheHashValue1(VARIABLEOID,
+											ObjectIdGetDatum(var->oid));
+
+	/* the value of variable is not known yet */
+	svar->is_valid = false;
+
+	if (var->eoxaction == VARIABLE_EOX_RESET ||
+			var->eoxaction == VARIABLE_EOX_DROP)
+		register_session_variable_xact_action(var->oid, RESET);
+}
+
+/*
+ * Try to search value in hash table. If it doesn't
+ * exist, then insert it (and calculate defexpr if it exists).
+ *
+ * As side efect this function acquires AccessShareLock on
+ * related session variable until commit.
+ */
+static SVariable
+prepare_variable_for_reading(Oid varid)
+{
+	SVariable svar;
+	Variable	var;
+	bool		found;
+
+	var.oid = InvalidOid;
+
+	if (!sessionvars)
+		create_sessionvars_hashtable();
+
+	/* Protect used session variable against drop until commit */
+	LockDatabaseObject(VariableRelationId, varid, 0, AccessShareLock);
+
+	svar = (SVariable) hash_search(sessionvars, &varid,
+										HASH_ENTER, &found);
+
+	/* Return content if it is available and valid */
+	if (found && svar->is_valid)
+		return svar;
+
+	/* We need to load defexpr. */
+	initVariable(&var, varid, false, false);
+
+	if (!found)
+		init_session_variable(svar, &var);
+
+	/* Raise an error when we cannot initialize variable correctly */
+	if (var.is_not_null && !var.defexpr)
+		ereport(ERROR,
+				(errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
+				 errmsg("null value is not allowed for NOT NULL session variable \"%s\"",
+						session_variable_get_name(varid)),
+				 errdetail("The session variable was not initialized yet.")));
+
+	if (svar->has_defexpr)
+	{
+		Datum		value = (Datum) 0;
+		bool		isnull;
+		EState	   *estate = NULL;
+		Expr	   *defexpr;
+		ExprState  *defexprs;
+		MemoryContext oldcxt;
+
+		/* Prepare default expr */
+		estate = CreateExecutorState();
+
+		oldcxt = MemoryContextSwitchTo(estate->es_query_cxt);
+
+		defexpr = expression_planner((Expr *) var.defexpr);
+		defexprs = ExecInitExpr(defexpr, NULL);
+		value = ExecEvalExprSwitchContext(defexprs,
+										  GetPerTupleExprContext(estate),
+										  &isnull);
+
+
+		/* Store result before releasing Executor memory */
+		set_session_variable(svar, value, isnull, svar->typid, true);
+
+		MemoryContextSwitchTo(oldcxt);
+
+		FreeExecutorState(estate);
+	}
+	else
+		set_session_variable(svar, (Datum) 0, true, svar->typid, true);
+
+	return svar;
+}
+
+/*
+ * Write value to variable. We expect secured access in this moment.
+ * We try not to break the previous value, if something is wrong.
+ *
+ * As side efect this function acquires AccessShareLock on
+ * related session variable until commit.
+ */
+void
+SetSessionVariable(Oid varid, Datum value, bool isNull, Oid typid)
+{
+	SVariable svar;
+	bool		found;
+
+	/* Protect used session variable against drop until commit */
+	LockDatabaseObject(VariableRelationId, varid, 0, AccessShareLock);
+
+	if (!sessionvars)
+		create_sessionvars_hashtable();
+
+	svar = (SVariable) hash_search(sessionvars, &varid,
+										HASH_ENTER, &found);
+
+	/* Initialize svar when not initialized or when stored value is null */
+	if (!found)
+	{
+		Variable	var;
+
+		/* don't need defexpr and acl here */
+		initVariable(&var, varid, false, true);
+		init_session_variable(svar, &var);
+	}
+
+	set_session_variable(svar, value, isNull, typid, false);
+}
+
+/*
+ * Write value to variable with security check.
+ * We try not to break the previous value, if something is wrong.
+ */
+void
+SetSessionVariableWithSecurityCheck(Oid varid, Datum value, bool isNull, Oid typid)
+{
+	AclResult	aclresult;
+
+	/*
+	 * Is possible to write to session variable?
+	 */
+	aclresult = pg_variable_aclcheck(varid, GetUserId(), ACL_WRITE);
+	if (aclresult != ACLCHECK_OK)
+		aclcheck_error(aclresult, OBJECT_VARIABLE, session_variable_get_name(varid));
+
+	SetSessionVariable(varid, value, isNull, typid);
+}
+
+/*
+ * Returns a copy of value of the session variable specified by varid
+ */
+Datum
+CopySessionVariable(Oid varid, bool *isNull, Oid *typid)
+{
+	SVariable svar;
+
+	svar = prepare_variable_for_reading(varid);
+	Assert(svar != NULL && svar->is_valid);
+
+	*isNull = svar->isnull;
+	*typid = svar->typid;
+
+	if (!svar->isnull)
+		return datumCopy(svar->value, svar->typbyval, svar->typlen);
+
+	return (Datum) 0;
+}
+
+/*
+ * Returns the value of the session variable specified by varid. Check correct
+ * result type. Optionally the result can be copied.
+ */
+Datum
+GetSessionVariable(Oid varid, bool *isNull, Oid expected_typid, bool copy)
+{
+	SVariable svar;
+	Datum		value;
+	bool		isnull;
+
+	svar = prepare_variable_for_reading(varid);
+	Assert(svar != NULL);
+
+	if (expected_typid != svar->typid)
+		elog(ERROR, "type of variable \"%s\" is different than expected",
+			 session_variable_get_name(varid));
+
+	value = svar->value;
+	isnull = svar->isnull;
+
+	*isNull = isnull;
+
+	if (!isnull && copy)
+		return datumCopy(value, svar->typbyval, svar->typlen);
+
+	return value;
+}
+
+
+/*
+ * Routines used for manipulation with session variables from
+ * SQL level
+ */
+
+/*
+ * Creates new variable - entry in pg_catalog.pg_variable table
+ *
+ * Used by CREATE VARIABLE command
+ */
+ObjectAddress
+DefineSessionVariable(ParseState *pstate, CreateSessionVarStmt *stmt)
+{
+	Oid			namespaceid;
+	AclResult	aclresult;
+	Oid			typid;
+	int32		typmod;
+	Oid			varowner = GetUserId();
+	Oid			collation;
+	Oid			typcollation;
+	ObjectAddress variable;
+
+	Node	   *cooked_default = NULL;
+
+	/*
+	 * Check consistency of arguments
+	 */
+	if (stmt->eoxaction == VARIABLE_EOX_DROP
+		&& stmt->variable->relpersistence != RELPERSISTENCE_TEMP)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+				 errmsg("ON COMMIT DROP can only be used on temporary variables")));
+
+	if (stmt->is_not_null && stmt->is_immutable && !stmt->defexpr)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+				 errmsg("IMMUTABLE NOT NULL variable requires default expression")));
+
+	namespaceid =
+		RangeVarGetAndCheckCreationNamespace(stmt->variable, NoLock, NULL);
+
+	typenameTypeIdAndMod(pstate, stmt->typeName, &typid, &typmod);
+	typcollation = get_typcollation(typid);
+
+	aclresult = pg_type_aclcheck(typid, GetUserId(), ACL_USAGE);
+	if (aclresult != ACLCHECK_OK)
+		aclcheck_error_type(aclresult, typid);
+
+	if (stmt->collClause)
+		collation = LookupCollation(pstate,
+									stmt->collClause->collname,
+									stmt->collClause->location);
+	else
+		collation = typcollation;;
+
+	/* Complain if COLLATE is applied to an uncollatable type */
+	if (OidIsValid(collation) && !OidIsValid(typcollation))
+		ereport(ERROR,
+				(errcode(ERRCODE_DATATYPE_MISMATCH),
+				 errmsg("collations are not supported by type %s",
+						format_type_be(typid)),
+				 parser_errposition(pstate, stmt->collClause->location)));
+
+	if (stmt->defexpr)
+	{
+		cooked_default = transformExpr(pstate, stmt->defexpr,
+									   EXPR_KIND_VARIABLE_DEFAULT);
+
+		cooked_default = coerce_to_specific_type(pstate,
+												 cooked_default, typid, "DEFAULT");
+		assign_expr_collations(pstate, cooked_default);
+	}
+
+	variable = VariableCreate(stmt->variable->relname,
+							  namespaceid,
+							  typid,
+							  typmod,
+							  varowner,
+							  collation,
+							  cooked_default,
+							  stmt->eoxaction,
+							  stmt->is_not_null,
+							  stmt->if_not_exists,
+							  stmt->is_immutable);
+
+	/*
+	 * We must bump the command counter to make the newly-created variable
+	 * tuple visible for any other operations.
+	 */
+	CommandCounterIncrement();
+
+	return variable;
+}
+
+/*
+ * Create new ON_COMMIT_DROP xact action. We have to drop
+ * ON COMMIT DROP variable, although this variable should not
+ * be used. So we need to register this action in CREATE VARIABLE
+ * time.
+ */
+void
+RegisterOnCommitDropSessionVariable(Oid varid)
+{
+	register_session_variable_xact_action(varid, ON_COMMIT_DROP);
+}
+
+/*
+ * Drop variable by OID. This routine doesn't try to remove
+ * the value of session variable immediately. It will be
+ * removed on transaction end in sync_sessionvars_xact_callback
+ * routine. This routine manipulate just with system catalog.
+ */
+void
+RemoveSessionVariable(Oid varid)
+{
+	Relation	rel;
+	HeapTuple	tup;
+
+	rel = table_open(VariableRelationId, RowExclusiveLock);
+
+	tup = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(varid));
+
+	if (!HeapTupleIsValid(tup))
+		elog(ERROR, "cache lookup failed for variable %u", varid);
+
+	CatalogTupleDelete(rel, &tup->t_self);
+
+	ReleaseSysCache(tup);
+
+	table_close(rel, RowExclusiveLock);
+
+	/*
+	 * we removed entry from sys catalog already, we should not to do
+	 * again at xact time,
+	 */
+	delete_session_variable_xact_action(varid, ON_COMMIT_DROP);
+
+	/*
+	 * and if this transaction or subtransaction will be committed,
+	 * we want to enforce variable cleaning. (we don't need to wait for
+	 * sinval message). The cleaning action for one session variable
+	 * can be repeated in the action list, and it doesn't do any problem
+	 * (so we don't need to ensure uniqueness).
+	 * We need separate action than RESET, because RESET
+	 * is executed on any transaction end, but we want to execute cleaning
+	 * only when current transaction will be commited.
+	 */
+	register_session_variable_xact_action(varid, ON_COMMIT_RESET);
+}
+
+/*
+ * Fast drop complete content of all session variables hash table.
+ * This is code for DISCARD VARIABLES command. This command
+ * cannot to run inside transaction, so we don't need to handle
+ * end of transaction actions.
+ */
+void
+ResetSessionVariables(void)
+{
+	/* Destroy hash table and reset related memory context */
+	if (sessionvars)
+	{
+		hash_destroy(sessionvars);
+		sessionvars = NULL;
+	}
+
+	/* Release memory allocated by session variables */
+	if (SVariableMemoryContext != NULL)
+		MemoryContextReset(SVariableMemoryContext);
+
+	/*
+	 * There are not any session variables, so trim
+	 * both xact action lists.
+	 */
+	list_free_deep(xact_drop_actions);
+	xact_drop_actions = NIL;
+
+	list_free_deep(xact_reset_actions);
+	xact_drop_actions = NIL;
+}
+
+/*
+ * Assign result of evaluated expression to session variable
+ */
+void
+ExecuteLetStmt(ParseState *pstate,
+			   LetStmt *stmt,
+			   ParamListInfo params,
+			   QueryEnvironment *queryEnv,
+			   QueryCompletion *qc)
+{
+	Query	   *query = castNode(Query, stmt->query);
+	List	   *rewritten;
+	DestReceiver *dest;
+	AclResult	aclresult;
+	PlannedStmt *plan;
+	QueryDesc  *queryDesc;
+	Oid			varid = query->resultVariable;
+
+	Assert(OidIsValid(varid));
+
+	/*
+	 * Is it allowed to write to session variable?
+	 */
+	aclresult = pg_variable_aclcheck(varid, GetUserId(), ACL_WRITE);
+	if (aclresult != ACLCHECK_OK)
+		aclcheck_error(aclresult, OBJECT_VARIABLE, session_variable_get_name(varid));
+
+	/* Create dest receiver for LET */
+	dest = CreateDestReceiver(DestVariable);
+	SetVariableDestReceiverParams(dest, varid);
+
+	/* run rewriter - can be used for replacement of DEFAULT node */
+	query = copyObject(query);
+
+	rewritten = QueryRewrite(query);
+
+	Assert(list_length(rewritten) == 1);
+
+	query = linitial_node(Query, rewritten);
+	Assert(query->commandType == CMD_SELECT);
+
+	/* plan the query */
+	plan = pg_plan_query(query, pstate->p_sourcetext,
+						 CURSOR_OPT_PARALLEL_OK, params);
+
+	/*
+	 * Use a snapshot with an updated command ID to ensure this query sees
+	 * results of any previously executed queries.  (This could only
+	 * matter if the planner executed an allegedly-stable function that
+	 * changed the database contents, but let's do it anyway to be
+	 * parallel to the EXPLAIN code path.)
+	 */
+	PushCopiedSnapshot(GetActiveSnapshot());
+	UpdateActiveSnapshotCommandId();
+
+	/* Create a QueryDesc, redirecting output to our tuple receiver */
+	queryDesc = CreateQueryDesc(plan, pstate->p_sourcetext,
+								GetActiveSnapshot(), InvalidSnapshot,
+								dest, params, queryEnv, 0);
+
+	/* call ExecutorStart to prepare the plan for execution */
+	ExecutorStart(queryDesc, 0);
+
+	/* run the plan to completion */
+	ExecutorRun(queryDesc, ForwardScanDirection, 2L, true);
+
+	/* save the rowcount if we're given a qc to fill */
+	if (qc)
+		SetQueryCompletion(qc, CMDTAG_LET, queryDesc->estate->es_processed);
+
+	/* and clean up */
+	ExecutorFinish(queryDesc);
+	ExecutorEnd(queryDesc);
+
+	FreeQueryDesc(queryDesc);
+
+	PopActiveSnapshot();
+}
+
+
+/*
+ * Implementation of drop or reset actions executed on session variables
+ * at transaction end time. We want to drop temporary session variables
+ * with clause ON COMMIT DROP, or we want to reset values of session variables
+ * with clause ON TRANSACTION END RESET or we want to clean (reset) memory
+ * allocated by values of dropped session variables.
+ */
+
+/*
+ * Register a session variable xact action.
+ */
+void
+register_session_variable_xact_action(Oid varid,
+									  SVariableXActAction action)
+{
+	SVariableXActActionItem *xact_ai;
+	MemoryContext oldcxt;
+
+	oldcxt = MemoryContextSwitchTo(CacheMemoryContext);
+
+	xact_ai = (SVariableXActActionItem *)
+							palloc(sizeof(SVariableXActActionItem));
+
+	xact_ai->varid = varid;
+	xact_ai->action = action;
+
+	xact_ai->creating_subid = GetCurrentSubTransactionId();
+	xact_ai->deleting_subid = InvalidSubTransactionId;
+
+	if (action == ON_COMMIT_DROP)
+		xact_drop_actions = lcons(xact_ai, xact_drop_actions);
+	else
+		xact_reset_actions = lcons(xact_ai, xact_reset_actions);
+
+	MemoryContextSwitchTo(oldcxt);
+}
+
+/*
+ * Remove variable from action list. In this moment,
+ * the action is just marked as deleted by setting
+ * deleting_subid.
+ */
+static void
+delete_session_variable_xact_action(Oid varid,
+									SVariableXActAction action)
+{
+	ListCell   *l;
+
+	Assert(action == ON_COMMIT_DROP);
+
+	foreach(l, xact_drop_actions)
+	{
+		SVariableXActActionItem *xact_ai =
+					(SVariableXActActionItem *) lfirst(l);
+
+		if (xact_ai->varid == varid && xact_ai->action == action)
+			xact_ai->deleting_subid = GetCurrentSubTransactionId();
+	}
+}
+
+/*
+ * Perform ON TRANSACTION END RESET or ON COMMIT DROP
+ * and COMMIT/ROLLBACK of transaction session variables.
+ */
+void
+AtPreEOXact_SessionVariable_on_xact_actions(bool isCommit)
+{
+	ListCell   *l;
+
+	foreach(l, xact_drop_actions)
+	{
+		SVariableXActActionItem *xact_ai =
+							(SVariableXActActionItem *) lfirst(l);
+
+		/* Iterate only over non dropped entries */
+		if (xact_ai->deleting_subid == InvalidSubTransactionId)
+		{
+			Assert(xact_ai->action == ON_COMMIT_DROP);
+
+			/*
+			 * ON COMMIT DROP is allowed only for temp session
+			 * variables. So we should explicitly delete only when
+			 * current transaction was committed. When it's rollback,
+			 * then session variable is removed automatically.
+			 */
+			if (isCommit)
+			{
+				ObjectAddress object;
+
+				object.classId = VariableRelationId;
+				object.objectId = xact_ai->varid;
+				object.objectSubId = 0;
+
+				/*
+				 * Since this is an automatic drop, rather than one
+				 * directly initiated by the user, we pass the
+				 * PERFORM_DELETION_INTERNAL flag.
+				 */
+				performDeletion(&object, DROP_CASCADE,
+								PERFORM_DELETION_INTERNAL |
+								PERFORM_DELETION_QUIETLY);
+			}
+		}
+	}
+
+	list_free_deep(xact_drop_actions);
+	xact_drop_actions = NIL;
+
+	foreach(l, xact_reset_actions)
+	{
+		SVariableXActActionItem *xact_ai =
+							(SVariableXActActionItem *) lfirst(l);
+
+		if (xact_ai->action == RECHECK)
+		{
+			/*
+			 * we can do recheck only in transactionn state
+			 * when we can touch system catalog. When transaction
+			 * is ending by ROLLBACK we are not in transaction state.
+			 */
+			if (IsTransactionState())
+			{
+				SVariable	svar;
+				bool		found;
+
+				svar = (SVariable) hash_search(sessionvars, &xact_ai->varid,
+											HASH_FIND, &found);
+
+				if (found)
+				{
+					HeapTuple	tp;
+
+					tp = SearchSysCache1(VARIABLEOID, ObjectIdGetDatum(svar->varid));
+
+					if (HeapTupleIsValid(tp))
+						ReleaseSysCache(tp);
+					else
+						free_session_variable(svar);
+				}
+
+				xact_reset_actions = foreach_delete_current(xact_reset_actions, l);
+			}
+		}
+		else
+		{
+			/*
+			 * We want to reset session variable (release it from
+			 * local memory) when RESET is required or when session
+			 * variable was removed explicitly (DROP VARIABLE) or
+			 * implicitly (ON COMMIT DROP). Explicit releasing should
+			 * be done only if the transaction is commited.
+			 */
+			if ((xact_ai->action == RESET) ||
+				(xact_ai->action == ON_COMMIT_RESET &&
+				 xact_ai->deleting_subid == InvalidSubTransactionId &&
+				 isCommit))
+				free_session_variable_varid(xact_ai->varid);
+
+			xact_reset_actions = foreach_delete_current(xact_reset_actions, l);
+		}
+	}
+
+	list_free_deep(xact_drop_actions);
+	list_free_deep(xact_reset_actions);
+
+	xact_drop_actions = NIL;
+	xact_reset_actions = NIL;
+}
+
+/*
+ * Post-subcommit or post-subabort cleanup of xact action list.
+ *
+ * During subabort, we can immediately remove entries created during this
+ * subtransaction.  During subcommit, just relabel entries marked during
+ * this subtransaction as being the parent's responsibility.
+ */
+void
+AtEOSubXact_SessionVariable_on_xact_actions(bool isCommit, SubTransactionId mySubid,
+											SubTransactionId parentSubid)
+{
+	ListCell   *cur_item;
+
+	foreach(cur_item, xact_drop_actions)
+	{
+		SVariableXActActionItem *xact_ai =
+								  (SVariableXActActionItem *) lfirst(cur_item);
+
+		if (!isCommit && xact_ai->creating_subid == mySubid)
+		{
+			/* cur_item must be removed */
+			xact_drop_actions = foreach_delete_current(xact_drop_actions, cur_item);
+			pfree(xact_ai);
+		}
+		else
+		{
+			/* cur_item must be preserved */
+			if (xact_ai->creating_subid == mySubid)
+				xact_ai->creating_subid = parentSubid;
+			if (xact_ai->deleting_subid == mySubid)
+				xact_ai->deleting_subid = isCommit ? parentSubid : InvalidSubTransactionId;
+		}
+	}
+
+	/*
+	 * Reset and recheck actions - cleaning memory should be used every time
+	 * (when the variable with short life cycle was used) and then
+	 * cannot be removed from xact action list.
+	 */
+	foreach(cur_item, xact_reset_actions)
+	{
+		SVariableXActActionItem *xact_ai =
+								  (SVariableXActActionItem *) lfirst(cur_item);
+
+		if (!isCommit &&
+			xact_ai->creating_subid == mySubid &&
+			xact_ai->action != RESET &&
+			xact_ai->action != RECHECK)
+		{
+			/* cur_item must be removed */
+			xact_reset_actions = foreach_delete_current(xact_reset_actions, cur_item);
+			pfree(xact_ai);
+		}
+		else
+		{
+			/* cur_item must be preserved */
+			if (xact_ai->creating_subid == mySubid)
+				xact_ai->creating_subid = parentSubid;
+			if (xact_ai->deleting_subid == mySubid)
+				xact_ai->deleting_subid = isCommit ? parentSubid : InvalidSubTransactionId;
+		}
+	}
+}
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 1f0654c2f5..171cd66929 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -12647,6 +12647,7 @@ ATExecAlterColumnType(AlteredTableInfo *tab, Relation rel,
 			case OCLASS_PUBLICATION_REL:
 			case OCLASS_SUBSCRIPTION:
 			case OCLASS_TRANSFORM:
+			case OCLASS_VARIABLE:
 
 				/*
 				 * We don't expect any of these sorts of objects to depend on
diff --git a/src/backend/executor/Makefile b/src/backend/executor/Makefile
index 11118d0ce0..71248a34f2 100644
--- a/src/backend/executor/Makefile
+++ b/src/backend/executor/Makefile
@@ -76,6 +76,7 @@ OBJS = \
 	nodeWindowAgg.o \
 	nodeWorktablescan.o \
 	spi.o \
+	svariableReceiver.o \
 	tqueue.o \
 	tstoreReceiver.o
 
diff --git a/src/backend/executor/execExpr.c b/src/backend/executor/execExpr.c
index 847357bf80..834fb4e606 100644
--- a/src/backend/executor/execExpr.c
+++ b/src/backend/executor/execExpr.c
@@ -33,6 +33,7 @@
 #include "access/nbtree.h"
 #include "catalog/objectaccess.h"
 #include "catalog/pg_type.h"
+#include "commands/session_variable.h"
 #include "executor/execExpr.h"
 #include "executor/nodeSubplan.h"
 #include "funcapi.h"
@@ -994,6 +995,60 @@ ExecInitExprRec(Expr *node, ExprState *state,
 						scratch.d.param.paramtype = param->paramtype;
 						ExprEvalPushStep(state, &scratch);
 						break;
+
+					case PARAM_VARIABLE:
+						{
+							int			es_num_session_variables = 0;
+							SessionVariableValue *es_session_variables = NULL;
+
+							if (state->parent && state->parent->state)
+							{
+								es_session_variables = state->parent->state->es_session_variables;
+								es_num_session_variables = state->parent->state->es_num_session_variables;
+							}
+
+							/*
+							 * We should use session variable buffer, when
+							 * it is available.
+							 */
+							if (es_session_variables)
+							{
+								SessionVariableValue *var;
+
+								/* check params, unexpected */
+								if (param->paramid >= es_num_session_variables)
+									elog(ERROR, "paramid of PARAM_VARIABLE param is out of range");
+
+								var = &es_session_variables[param->paramid];
+
+								/* unexpected */
+								if (var->typid != param->paramtype)
+									elog(ERROR, "type of buffered value is different than PARAM_VARIABLE type");
+
+								/*
+								 * In this case, the parameter is like a
+								 * constant
+								 */
+								scratch.opcode = EEOP_CONST;
+								scratch.d.constval.value = var->value;
+								scratch.d.constval.isnull = var->isnull;
+								ExprEvalPushStep(state, &scratch);
+							}
+							else
+							{
+								/*
+								 * When we have no full PlanState (plpgsql
+								 * simple expr evaluation), then we should
+								 * use direct access.
+								 */
+								scratch.opcode = EEOP_PARAM_VARIABLE;
+								scratch.d.vparam.varid = param->paramvarid;
+								scratch.d.vparam.vartype = param->paramtype;
+								ExprEvalPushStep(state, &scratch);
+							}
+						}
+						break;
+
 					case PARAM_EXTERN:
 
 						/*
diff --git a/src/backend/executor/execExprInterp.c b/src/backend/executor/execExprInterp.c
index d6f7d7c2d7..ee9cef5983 100644
--- a/src/backend/executor/execExprInterp.c
+++ b/src/backend/executor/execExprInterp.c
@@ -59,6 +59,7 @@
 #include "access/heaptoast.h"
 #include "catalog/pg_type.h"
 #include "commands/sequence.h"
+#include "commands/session_variable.h"
 #include "executor/execExpr.h"
 #include "executor/nodeSubplan.h"
 #include "funcapi.h"
@@ -444,6 +445,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 		&&CASE_EEOP_PARAM_EXEC,
 		&&CASE_EEOP_PARAM_EXTERN,
 		&&CASE_EEOP_PARAM_CALLBACK,
+		&&CASE_EEOP_PARAM_VARIABLE,
 		&&CASE_EEOP_CASE_TESTVAL,
 		&&CASE_EEOP_MAKE_READONLY,
 		&&CASE_EEOP_IOCOERCE,
@@ -1078,6 +1080,16 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			EEO_NEXT();
 		}
 
+		EEO_CASE(EEOP_PARAM_VARIABLE)
+		{
+			/* direct access to session variable (without buffering) */
+			*op->resvalue = GetSessionVariable(op->d.vparam.varid,
+											  op->resnull,
+											  op->d.vparam.vartype,
+											  true);
+			EEO_NEXT();
+		}
+
 		EEO_CASE(EEOP_CASE_TESTVAL)
 		{
 			/*
diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index 549d9eb696..0cd5fbfa8a 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -45,10 +45,13 @@
 #include "access/xact.h"
 #include "catalog/namespace.h"
 #include "catalog/pg_publication.h"
+#include "catalog/pg_variable.h"
 #include "commands/matview.h"
 #include "commands/trigger.h"
+#include "commands/session_variable.h"
 #include "executor/execdebug.h"
 #include "executor/nodeSubplan.h"
+#include "executor/svariableReceiver.h"
 #include "foreign/fdwapi.h"
 #include "jit/jit.h"
 #include "mb/pg_wchar.h"
@@ -199,6 +202,52 @@ standard_ExecutorStart(QueryDesc *queryDesc, int eflags)
 	Assert(queryDesc->sourceText != NULL);
 	estate->es_sourceText = queryDesc->sourceText;
 
+	/*
+	 * Prepare session variables, if not prepared in queryDesc
+	 */
+	if (queryDesc->num_session_variables > 0)
+	{
+		/*
+		 * link shared memory with working copy of session variable's values
+		 * used in this query. This access is used by parallel query executor's
+		 * workers.
+		 */
+		estate->es_session_variables = queryDesc->session_variables;
+		estate->es_num_session_variables = queryDesc->num_session_variables;
+	}
+	else if (queryDesc->plannedstmt->sessionVariables)
+	{
+		ListCell   *lc;
+		int			nSessionVariables;
+		int			i = 0;
+
+		nSessionVariables = list_length(queryDesc->plannedstmt->sessionVariables);
+
+		/* Create buffer used for session variables */
+		estate->es_session_variables = (SessionVariableValue *)
+			palloc(nSessionVariables * sizeof(SessionVariableValue));
+
+		foreach(lc, queryDesc->plannedstmt->sessionVariables)
+		{
+			AclResult	aclresult;
+			Oid			varid = lfirst_oid(lc);
+
+			aclresult = pg_variable_aclcheck(varid, GetUserId(), ACL_READ);
+			if (aclresult != ACLCHECK_OK)
+				aclcheck_error(aclresult, OBJECT_VARIABLE,
+							   session_variable_get_name(varid));
+
+			estate->es_session_variables[i].varid = varid;
+			estate->es_session_variables[i].value = CopySessionVariable(varid,
+																	  &estate->es_session_variables[i].isnull,
+																	  &estate->es_session_variables[i].typid);
+
+			i++;
+		}
+
+		estate->es_num_session_variables = nSessionVariables;
+	}
+
 	/*
 	 * Fill in the query environment, if any, from queryDesc.
 	 */
diff --git a/src/backend/executor/execParallel.c b/src/backend/executor/execParallel.c
index 5dd8ab7db2..863f9fbcf5 100644
--- a/src/backend/executor/execParallel.c
+++ b/src/backend/executor/execParallel.c
@@ -12,8 +12,9 @@
  * workers and ensuring that their state generally matches that of the
  * leader; see src/backend/access/transam/README.parallel for details.
  * However, we must save and restore relevant executor state, such as
- * any ParamListInfo associated with the query, buffer/WAL usage info, and
- * the actual plan to be passed down to the worker.
+ * any ParamListInfo associated with the query, buffer/WAL usage info,
+ * session variables buffer, and the actual plan to be passed down to
+ * the worker.
  *
  * IDENTIFICATION
  *	  src/backend/executor/execParallel.c
@@ -66,6 +67,7 @@
 #define PARALLEL_KEY_QUERY_TEXT		UINT64CONST(0xE000000000000008)
 #define PARALLEL_KEY_JIT_INSTRUMENTATION UINT64CONST(0xE000000000000009)
 #define PARALLEL_KEY_WAL_USAGE			UINT64CONST(0xE00000000000000A)
+#define PARALLEL_KEY_SESSION_VARIABLES	UINT64CONST(0xE00000000000000B)
 
 #define PARALLEL_TUPLE_QUEUE_SIZE		65536
 
@@ -140,6 +142,12 @@ static bool ExecParallelRetrieveInstrumentation(PlanState *planstate,
 /* Helper function that runs in the parallel worker. */
 static DestReceiver *ExecParallelGetReceiver(dsm_segment *seg, shm_toc *toc);
 
+/* Helper functions that can pass values of session variables */
+static Size EstimateSessionVariables(EState *estate);
+static void SerializeSessionVariables(EState *estate, char **start_address);
+static SessionVariableValue * RestoreSessionVariables(char **start_address,
+													int *num_session_variables);
+
 /*
  * Create a serialized representation of the plan to be sent to each worker.
  */
@@ -597,6 +605,7 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate,
 	char	   *pstmt_data;
 	char	   *pstmt_space;
 	char	   *paramlistinfo_space;
+	char	   *session_variables_space;
 	BufferUsage *bufusage_space;
 	WalUsage   *walusage_space;
 	SharedExecutorInstrumentation *instrumentation = NULL;
@@ -606,6 +615,7 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate,
 	int			instrumentation_len = 0;
 	int			jit_instrumentation_len = 0;
 	int			instrument_offset = 0;
+	int			session_variables_len = 0;
 	Size		dsa_minsize = dsa_minimum_size();
 	char	   *query_string;
 	int			query_len;
@@ -661,6 +671,11 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate,
 	shm_toc_estimate_chunk(&pcxt->estimator, paramlistinfo_len);
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
 
+	/* Estimate space for serialized session variables. */
+	session_variables_len = EstimateSessionVariables(estate);
+	shm_toc_estimate_chunk(&pcxt->estimator, session_variables_len);
+	shm_toc_estimate_keys(&pcxt->estimator, 1);
+
 	/*
 	 * Estimate space for BufferUsage.
 	 *
@@ -755,6 +770,11 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate,
 	shm_toc_insert(pcxt->toc, PARALLEL_KEY_PARAMLISTINFO, paramlistinfo_space);
 	SerializeParamList(estate->es_param_list_info, &paramlistinfo_space);
 
+	/* Store serialized session variables. */
+	session_variables_space = shm_toc_allocate(pcxt->toc, session_variables_len);
+	shm_toc_insert(pcxt->toc, PARALLEL_KEY_SESSION_VARIABLES, session_variables_space);
+	SerializeSessionVariables(estate, &session_variables_space);
+
 	/* Allocate space for each worker's BufferUsage; no need to initialize. */
 	bufusage_space = shm_toc_allocate(pcxt->toc,
 									  mul_size(sizeof(BufferUsage), pcxt->nworkers));
@@ -1402,6 +1422,7 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
 	SharedJitInstrumentation *jit_instrumentation;
 	int			instrument_options = 0;
 	void	   *area_space;
+	char	   *sessionvariable_space;
 	dsa_area   *area;
 	ParallelWorkerContext pwcxt;
 
@@ -1427,6 +1448,14 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
 	area_space = shm_toc_lookup(toc, PARALLEL_KEY_DSA, false);
 	area = dsa_attach_in_place(area_space, seg);
 
+	/* Reconstruct session variables. */
+	sessionvariable_space = shm_toc_lookup(toc,
+										  PARALLEL_KEY_SESSION_VARIABLES,
+										  false);
+	queryDesc->session_variables =
+		RestoreSessionVariables(&sessionvariable_space,
+							   &queryDesc->num_session_variables);
+
 	/* Start up the executor */
 	queryDesc->plannedstmt->jitFlags = fpes->jit_flags;
 	ExecutorStart(queryDesc, fpes->eflags);
@@ -1496,3 +1525,118 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
 	FreeQueryDesc(queryDesc);
 	receiver->rDestroy(receiver);
 }
+
+/*
+ * Estimate the amount of space required to serialize a
+ * session variable.
+ */
+static Size
+EstimateSessionVariables(EState *estate)
+{
+	int			i;
+	Size		sz = sizeof(int);
+
+	if (estate->es_session_variables == NULL)
+		return sz;
+
+	for (i = 0; i < estate->es_num_session_variables; i++)
+	{
+		SessionVariableValue *svarval;
+		Oid			typeOid;
+		int16		typLen;
+		bool		typByVal;
+
+		svarval = &estate->es_session_variables[i];
+
+		typeOid = svarval->typid;
+
+		sz = add_size(sz, sizeof(Oid)); /* space for type OID */
+
+		/* space for datum/isnull */
+		Assert(OidIsValid(typeOid));
+		get_typlenbyval(typeOid, &typLen, &typByVal);
+
+		sz = add_size(sz,
+					  datumEstimateSpace(svarval->value, svarval->isnull, typByVal, typLen));
+	}
+
+	return sz;
+}
+
+/*
+ * Serialize a session variables buffer into caller-provided storage.
+ *
+ * We write the number of parameters first, as a 4-byte integer, and then
+ * write details for each parameter in turn.  The details for each parameter
+ * consist of a 4-byte type OID, and then the datum as serialized by
+ * datumSerialize().  The caller is responsible for ensuring that there is
+ * enough storage to store the number of bytes that will be written; use
+ * EstimateSessionVariables to find out how many will be needed.
+ * *start_address is updated to point to the byte immediately following those
+ * written.
+ *
+ * RestoreSessionVariables can be used to recreate a session variable buffer
+ * based on the serialized representation;
+ */
+static void
+SerializeSessionVariables(EState *estate, char **start_address)
+{
+	int			nparams;
+	int			i;
+
+	/* Write number of parameters. */
+	nparams = estate->es_num_session_variables;
+	memcpy(*start_address, &nparams, sizeof(int));
+	*start_address += sizeof(int);
+
+	/* Write each parameter in turn. */
+	for (i = 0; i < nparams; i++)
+	{
+		SessionVariableValue *svarval;
+		Oid			typeOid;
+		int16		typLen;
+		bool		typByVal;
+
+		svarval = &estate->es_session_variables[i];
+		typeOid = svarval->typid;
+
+		/* Write type OID. */
+		memcpy(*start_address, &typeOid, sizeof(Oid));
+		*start_address += sizeof(Oid);
+
+		Assert(OidIsValid(typeOid));
+		get_typlenbyval(typeOid, &typLen, &typByVal);
+
+		datumSerialize(svarval->value, svarval->isnull, typByVal, typLen,
+					   start_address);
+	}
+}
+
+static SessionVariableValue *
+RestoreSessionVariables(char **start_address, int *num_session_variables)
+{
+	SessionVariableValue *session_variables;
+	int			i;
+	int			nparams;
+
+	memcpy(&nparams, *start_address, sizeof(int));
+	*start_address += sizeof(int);
+
+	*num_session_variables = nparams;
+	session_variables = (SessionVariableValue *)
+		palloc(nparams * sizeof(SessionVariableValue));
+
+	for (i = 0; i < nparams; i++)
+	{
+		SessionVariableValue *svarval = &session_variables[i];
+
+		/* Read type OID. */
+		memcpy(&svarval->typid, *start_address, sizeof(Oid));
+		*start_address += sizeof(Oid);
+
+		/* Read datum/isnull. */
+		svarval->value = datumRestore(start_address, &svarval->isnull);
+	}
+
+	return session_variables;
+}
diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index c93f90de9b..b78ac20aae 100644
--- a/src/backend/executor/spi.c
+++ b/src/backend/executor/spi.c
@@ -2867,6 +2867,9 @@ _SPI_error_callback(void *arg)
 			case RAW_PARSE_PLPGSQL_ASSIGN3:
 				errcontext("PL/pgSQL assignment \"%s\"", query);
 				break;
+			case RAW_PARSE_PLPGSQL_LET:
+				errcontext("LET statement \"%s\"", query);
+				break;
 			default:
 				errcontext("SQL statement \"%s\"", query);
 				break;
diff --git a/src/backend/executor/svariableReceiver.c b/src/backend/executor/svariableReceiver.c
new file mode 100644
index 0000000000..66b8b6ec8c
--- /dev/null
+++ b/src/backend/executor/svariableReceiver.c
@@ -0,0 +1,145 @@
+/*-------------------------------------------------------------------------
+ *
+ * svariableReceiver.c
+ *	  An implementation of DestReceiver that stores the result value in
+ *	  a session variable.
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/executor/svariableReceiver.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "access/detoast.h"
+#include "executor/svariableReceiver.h"
+#include "commands/session_variable.h"
+
+typedef struct
+{
+	DestReceiver pub;
+	Oid			varid;
+	Oid			typid;
+	int32		typmod;
+	int			typlen;
+	int			slot_offset;
+	int			rows;
+}			svariableState;
+
+
+/*
+ * Prepare to receive tuples from executor.
+ */
+static void
+svariableStartupReceiver(DestReceiver *self, int operation, TupleDesc typeinfo)
+{
+	svariableState *myState = (svariableState *) self;
+	int			natts = typeinfo->natts;
+	int			outcols = 0;
+	int			i;
+
+	for (i = 0; i < natts; i++)
+	{
+		Form_pg_attribute attr = TupleDescAttr(typeinfo, i);
+
+		if (attr->attisdropped)
+			continue;
+
+		if (++outcols > 1)
+			elog(ERROR, "svariable DestReceiver can take only one attribute");
+
+		myState->typid = attr->atttypid;
+		myState->typmod = attr->atttypmod;
+		myState->typlen = attr->attlen;
+		myState->slot_offset = i;
+	}
+
+	myState->rows = 0;
+}
+
+/*
+ * Receive a tuple from the executor and store it in session variable.
+ */
+static bool
+svariableReceiveSlot(TupleTableSlot *slot, DestReceiver *self)
+{
+	svariableState *myState = (svariableState *) self;
+	Datum		value;
+	bool		isnull;
+	bool		freeval = false;
+
+	/* Make sure the tuple is fully deconstructed */
+	slot_getallattrs(slot);
+
+	value = slot->tts_values[myState->slot_offset];
+	isnull = slot->tts_isnull[myState->slot_offset];
+
+	if (myState->typlen == -1 && !isnull && VARATT_IS_EXTERNAL(DatumGetPointer(value)))
+	{
+		value = PointerGetDatum(detoast_external_attr((struct varlena *)
+													  DatumGetPointer(value)));
+		freeval = true;
+	}
+
+	SetSessionVariable(myState->varid, value, isnull, myState->typid);
+
+	if (freeval)
+		pfree(DatumGetPointer(value));
+
+	return true;
+}
+
+/*
+ * Clean up at end of an executor run
+ */
+static void
+svariableShutdownReceiver(DestReceiver *self)
+{
+	/* Do nothing */
+}
+
+/*
+ * Destroy receiver when done with it
+ */
+static void
+svariableDestroyReceiver(DestReceiver *self)
+{
+	pfree(self);
+}
+
+/*
+ * Initially create a DestReceiver object.
+ */
+DestReceiver *
+CreateVariableDestReceiver(void)
+{
+	svariableState *self = (svariableState *) palloc0(sizeof(svariableState));
+
+	self->pub.receiveSlot = svariableReceiveSlot;
+	self->pub.rStartup = svariableStartupReceiver;
+	self->pub.rShutdown = svariableShutdownReceiver;
+	self->pub.rDestroy = svariableDestroyReceiver;
+	self->pub.mydest = DestVariable;
+
+	/* private fields will be set by SetVariableDestReceiverParams */
+
+	return (DestReceiver *) self;
+}
+
+/*
+ * Set parameters for a VariableDestReceiver
+ */
+void
+SetVariableDestReceiverParams(DestReceiver *self, Oid varid)
+{
+	svariableState *myState = (svariableState *) self;
+
+	Assert(myState->pub.mydest == DestVariable);
+	Assert(OidIsValid(varid));
+
+	myState->varid = varid;
+}
diff --git a/src/backend/jit/llvm/llvmjit_expr.c b/src/backend/jit/llvm/llvmjit_expr.c
index bd86f546d7..e26f3e92e5 100644
--- a/src/backend/jit/llvm/llvmjit_expr.c
+++ b/src/backend/jit/llvm/llvmjit_expr.c
@@ -1073,6 +1073,12 @@ llvm_compile_expr(ExprState *state)
 				LLVMBuildBr(b, opblocks[opno + 1]);
 				break;
 
+			case EEOP_PARAM_VARIABLE:
+				build_EvalXFunc(b, mod, "ExecEvalParamVariable",
+								v_state, op, v_econtext);
+				LLVMBuildBr(b, opblocks[opno + 1]);
+				break;
+
 			case EEOP_PARAM_CALLBACK:
 				{
 					LLVMTypeRef v_functype;
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index 90b5da51c9..38fd27ca4a 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -106,6 +106,7 @@ _copyPlannedStmt(const PlannedStmt *from)
 	COPY_NODE_FIELD(invalItems);
 	COPY_NODE_FIELD(paramExecTypes);
 	COPY_NODE_FIELD(utilityStmt);
+	COPY_NODE_FIELD(sessionVariables);
 	COPY_LOCATION_FIELD(stmt_location);
 	COPY_SCALAR_FIELD(stmt_len);
 
@@ -1509,6 +1510,7 @@ _copyParam(const Param *from)
 	COPY_SCALAR_FIELD(paramtype);
 	COPY_SCALAR_FIELD(paramtypmod);
 	COPY_SCALAR_FIELD(paramcollid);
+	COPY_SCALAR_FIELD(paramvarid);
 	COPY_LOCATION_FIELD(location);
 
 	return newnode;
@@ -3170,6 +3172,7 @@ _copyQuery(const Query *from)
 	COPY_SCALAR_FIELD(canSetTag);
 	COPY_NODE_FIELD(utilityStmt);
 	COPY_SCALAR_FIELD(resultRelation);
+	COPY_SCALAR_FIELD(resultVariable);
 	COPY_SCALAR_FIELD(hasAggs);
 	COPY_SCALAR_FIELD(hasWindowFuncs);
 	COPY_SCALAR_FIELD(hasTargetSRFs);
@@ -3180,6 +3183,7 @@ _copyQuery(const Query *from)
 	COPY_SCALAR_FIELD(hasForUpdate);
 	COPY_SCALAR_FIELD(hasRowSecurity);
 	COPY_SCALAR_FIELD(isReturn);
+	COPY_SCALAR_FIELD(hasSessionVariables);
 	COPY_NODE_FIELD(cteList);
 	COPY_NODE_FIELD(rtable);
 	COPY_NODE_FIELD(jointree);
@@ -3293,6 +3297,20 @@ _copySelectStmt(const SelectStmt *from)
 	return newnode;
 }
 
+static LetStmt *
+_copyLetStmt(const LetStmt * from)
+{
+	LetStmt    *newnode = makeNode(LetStmt);
+
+	COPY_NODE_FIELD(target);
+	COPY_NODE_FIELD(query);
+	COPY_SCALAR_FIELD(set_default);
+	COPY_SCALAR_FIELD(plpgsql_mode);
+	COPY_LOCATION_FIELD(location);
+
+	return newnode;
+}
+
 static SetOperationStmt *
 _copySetOperationStmt(const SetOperationStmt *from)
 {
@@ -4907,6 +4925,23 @@ _copyDropSubscriptionStmt(const DropSubscriptionStmt *from)
 	return newnode;
 }
 
+static CreateSessionVarStmt *
+_copyCreateSessionVarStmt(const CreateSessionVarStmt *from)
+{
+	CreateSessionVarStmt *newnode = makeNode(CreateSessionVarStmt);
+
+	COPY_NODE_FIELD(variable);
+	COPY_NODE_FIELD(typeName);
+	COPY_NODE_FIELD(collClause);
+	COPY_NODE_FIELD(defexpr);
+	COPY_SCALAR_FIELD(eoxaction);
+	COPY_SCALAR_FIELD(if_not_exists);
+	COPY_SCALAR_FIELD(is_not_null);
+	COPY_SCALAR_FIELD(is_immutable);
+
+	return newnode;
+}
+
 /* ****************************************************************
  *					extensible.h copy functions
  * ****************************************************************
@@ -5423,6 +5458,9 @@ copyObjectImpl(const void *from)
 		case T_SelectStmt:
 			retval = _copySelectStmt(from);
 			break;
+		case T_LetStmt:
+			retval = _copyLetStmt(from);
+			break;
 		case T_SetOperationStmt:
 			retval = _copySetOperationStmt(from);
 			break;
@@ -5768,6 +5806,9 @@ copyObjectImpl(const void *from)
 		case T_DropSubscriptionStmt:
 			retval = _copyDropSubscriptionStmt(from);
 			break;
+		case  T_CreateSessionVarStmt:
+			retval = _copyCreateSessionVarStmt(from);
+			break;
 		case T_A_Expr:
 			retval = _copyA_Expr(from);
 			break;
diff --git a/src/backend/nodes/equalfuncs.c b/src/backend/nodes/equalfuncs.c
index 06345da3ba..86b3488148 100644
--- a/src/backend/nodes/equalfuncs.c
+++ b/src/backend/nodes/equalfuncs.c
@@ -215,6 +215,7 @@ _equalParam(const Param *a, const Param *b)
 	COMPARE_SCALAR_FIELD(paramtype);
 	COMPARE_SCALAR_FIELD(paramtypmod);
 	COMPARE_SCALAR_FIELD(paramcollid);
+	COMPARE_SCALAR_FIELD(paramvarid);
 	COMPARE_LOCATION_FIELD(location);
 
 	return true;
@@ -980,6 +981,7 @@ _equalQuery(const Query *a, const Query *b)
 	COMPARE_SCALAR_FIELD(canSetTag);
 	COMPARE_NODE_FIELD(utilityStmt);
 	COMPARE_SCALAR_FIELD(resultRelation);
+	COMPARE_SCALAR_FIELD(resultVariable);
 	COMPARE_SCALAR_FIELD(hasAggs);
 	COMPARE_SCALAR_FIELD(hasWindowFuncs);
 	COMPARE_SCALAR_FIELD(hasTargetSRFs);
@@ -990,6 +992,7 @@ _equalQuery(const Query *a, const Query *b)
 	COMPARE_SCALAR_FIELD(hasForUpdate);
 	COMPARE_SCALAR_FIELD(hasRowSecurity);
 	COMPARE_SCALAR_FIELD(isReturn);
+	COMPARE_SCALAR_FIELD(hasSessionVariables);
 	COMPARE_NODE_FIELD(cteList);
 	COMPARE_NODE_FIELD(rtable);
 	COMPARE_NODE_FIELD(jointree);
@@ -1093,6 +1096,17 @@ _equalSelectStmt(const SelectStmt *a, const SelectStmt *b)
 	return true;
 }
 
+static bool
+_equalLetStmt(const LetStmt * a, const LetStmt * b)
+{
+	COMPARE_NODE_FIELD(target);
+	COMPARE_NODE_FIELD(query);
+	COMPARE_SCALAR_FIELD(set_default);
+	COMPARE_SCALAR_FIELD(plpgsql_mode);
+
+	return true;
+}
+
 static bool
 _equalSetOperationStmt(const SetOperationStmt *a, const SetOperationStmt *b)
 {
@@ -2377,6 +2391,22 @@ _equalDropSubscriptionStmt(const DropSubscriptionStmt *a,
 	return true;
 }
 
+static bool
+_equalCreateSessionVarStmt(const CreateSessionVarStmt *a,
+						  const CreateSessionVarStmt *b)
+{
+	COMPARE_NODE_FIELD(variable);
+	COMPARE_NODE_FIELD(typeName);
+	COMPARE_NODE_FIELD(collClause);
+	COMPARE_NODE_FIELD(defexpr);
+	COMPARE_SCALAR_FIELD(eoxaction);
+	COMPARE_SCALAR_FIELD(if_not_exists);
+	COMPARE_SCALAR_FIELD(is_not_null);
+	COMPARE_SCALAR_FIELD(is_immutable);
+
+	return true;
+}
+
 static bool
 _equalCreatePolicyStmt(const CreatePolicyStmt *a, const CreatePolicyStmt *b)
 {
@@ -3420,6 +3450,9 @@ equal(const void *a, const void *b)
 		case T_SelectStmt:
 			retval = _equalSelectStmt(a, b);
 			break;
+		case T_LetStmt:
+			retval = _equalLetStmt(a, b);
+			break;
 		case T_SetOperationStmt:
 			retval = _equalSetOperationStmt(a, b);
 			break;
@@ -3765,6 +3798,9 @@ equal(const void *a, const void *b)
 		case T_DropSubscriptionStmt:
 			retval = _equalDropSubscriptionStmt(a, b);
 			break;
+		case T_CreateSessionVarStmt:
+			retval = _equalCreateSessionVarStmt(a, b);
+			break;
 		case T_A_Expr:
 			retval = _equalA_Expr(a, b);
 			break;
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 2b0236937a..9ed32aa987 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -324,6 +324,7 @@ _outPlannedStmt(StringInfo str, const PlannedStmt *node)
 	WRITE_NODE_FIELD(invalItems);
 	WRITE_NODE_FIELD(paramExecTypes);
 	WRITE_NODE_FIELD(utilityStmt);
+	WRITE_NODE_FIELD(sessionVariables);
 	WRITE_LOCATION_FIELD(stmt_location);
 	WRITE_INT_FIELD(stmt_len);
 }
@@ -1167,6 +1168,7 @@ _outParam(StringInfo str, const Param *node)
 	WRITE_OID_FIELD(paramtype);
 	WRITE_INT_FIELD(paramtypmod);
 	WRITE_OID_FIELD(paramcollid);
+	WRITE_OID_FIELD(paramvarid);
 	WRITE_LOCATION_FIELD(location);
 }
 
@@ -2280,6 +2282,7 @@ _outPlannerGlobal(StringInfo str, const PlannerGlobal *node)
 	WRITE_NODE_FIELD(relationOids);
 	WRITE_NODE_FIELD(invalItems);
 	WRITE_NODE_FIELD(paramExecTypes);
+	WRITE_NODE_FIELD(sessionVariables);
 	WRITE_UINT_FIELD(lastPHId);
 	WRITE_UINT_FIELD(lastRowMarkId);
 	WRITE_INT_FIELD(lastPlanNodeId);
@@ -2340,6 +2343,7 @@ _outPlannerInfo(StringInfo str, const PlannerInfo *node)
 	WRITE_BOOL_FIELD(hasPseudoConstantQuals);
 	WRITE_BOOL_FIELD(hasAlternativeSubPlans);
 	WRITE_BOOL_FIELD(hasRecursion);
+	WRITE_BOOL_FIELD(hasSessionVariables);
 	WRITE_INT_FIELD(wt_param_id);
 	WRITE_BITMAPSET_FIELD(curOuterRels);
 	WRITE_NODE_FIELD(curOuterParams);
@@ -2875,6 +2879,18 @@ _outPLAssignStmt(StringInfo str, const PLAssignStmt *node)
 	WRITE_LOCATION_FIELD(location);
 }
 
+static void
+_outLetStmt(StringInfo str, const LetStmt * node)
+{
+	WRITE_NODE_TYPE("LET");
+
+	WRITE_NODE_FIELD(target);
+	WRITE_NODE_FIELD(query);
+	WRITE_BOOL_FIELD(set_default);
+	WRITE_BOOL_FIELD(plpgsql_mode);
+	WRITE_LOCATION_FIELD(location);
+}
+
 static void
 _outFuncCall(StringInfo str, const FuncCall *node)
 {
@@ -3055,6 +3071,7 @@ _outQuery(StringInfo str, const Query *node)
 			case T_IndexStmt:
 			case T_NotifyStmt:
 			case T_DeclareCursorStmt:
+			case T_LetStmt:
 				WRITE_NODE_FIELD(utilityStmt);
 				break;
 			default:
@@ -3066,6 +3083,7 @@ _outQuery(StringInfo str, const Query *node)
 		appendStringInfoString(str, " :utilityStmt <>");
 
 	WRITE_INT_FIELD(resultRelation);
+	WRITE_OID_FIELD(resultVariable);
 	WRITE_BOOL_FIELD(hasAggs);
 	WRITE_BOOL_FIELD(hasWindowFuncs);
 	WRITE_BOOL_FIELD(hasTargetSRFs);
@@ -3076,6 +3094,7 @@ _outQuery(StringInfo str, const Query *node)
 	WRITE_BOOL_FIELD(hasForUpdate);
 	WRITE_BOOL_FIELD(hasRowSecurity);
 	WRITE_BOOL_FIELD(isReturn);
+	WRITE_BOOL_FIELD(hasSessionVariables);
 	WRITE_NODE_FIELD(cteList);
 	WRITE_NODE_FIELD(rtable);
 	WRITE_NODE_FIELD(jointree);
@@ -4385,6 +4404,9 @@ outNode(StringInfo str, const void *obj)
 			case T_PLAssignStmt:
 				_outPLAssignStmt(str, obj);
 				break;
+			case T_LetStmt:
+				_outLetStmt(str, obj);
+				break;
 			case T_ColumnDef:
 				_outColumnDef(str, obj);
 				break;
diff --git a/src/backend/nodes/readfuncs.c b/src/backend/nodes/readfuncs.c
index 3f68f7c18d..11933580aa 100644
--- a/src/backend/nodes/readfuncs.c
+++ b/src/backend/nodes/readfuncs.c
@@ -252,6 +252,7 @@ _readQuery(void)
 	READ_BOOL_FIELD(canSetTag);
 	READ_NODE_FIELD(utilityStmt);
 	READ_INT_FIELD(resultRelation);
+	READ_OID_FIELD(resultVariable);
 	READ_BOOL_FIELD(hasAggs);
 	READ_BOOL_FIELD(hasWindowFuncs);
 	READ_BOOL_FIELD(hasTargetSRFs);
@@ -262,6 +263,7 @@ _readQuery(void)
 	READ_BOOL_FIELD(hasForUpdate);
 	READ_BOOL_FIELD(hasRowSecurity);
 	READ_BOOL_FIELD(isReturn);
+	READ_BOOL_FIELD(hasSessionVariables);
 	READ_NODE_FIELD(cteList);
 	READ_NODE_FIELD(rtable);
 	READ_NODE_FIELD(jointree);
@@ -626,6 +628,7 @@ _readParam(void)
 	READ_OID_FIELD(paramtype);
 	READ_INT_FIELD(paramtypmod);
 	READ_OID_FIELD(paramcollid);
+	READ_OID_FIELD(paramvarid);
 	READ_LOCATION_FIELD(location);
 
 	READ_DONE();
@@ -1597,6 +1600,7 @@ _readPlannedStmt(void)
 	READ_NODE_FIELD(invalItems);
 	READ_NODE_FIELD(paramExecTypes);
 	READ_NODE_FIELD(utilityStmt);
+	READ_NODE_FIELD(sessionVariables);
 	READ_LOCATION_FIELD(stmt_location);
 	READ_INT_FIELD(stmt_len);
 
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index bd09f85aea..8c832d9de7 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -316,6 +316,7 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
 	glob->lastPlanNodeId = 0;
 	glob->transientPlan = false;
 	glob->dependsOnRole = false;
+	glob->sessionVariables = NIL;
 
 	/*
 	 * Assess whether it's feasible to use parallel mode for this query. We
@@ -529,6 +530,7 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
 	result->paramExecTypes = glob->paramExecTypes;
 	/* utilityStmt should be null, but we might as well copy it */
 	result->utilityStmt = parse->utilityStmt;
+	result->sessionVariables = glob->sessionVariables;
 	result->stmt_location = parse->stmt_location;
 	result->stmt_len = parse->stmt_len;
 
@@ -678,6 +680,12 @@ subquery_planner(PlannerGlobal *glob, Query *parse,
 	 */
 	pull_up_subqueries(root);
 
+	/*
+	 * Check if some subquery uses session variable. Flag hasSessionVariables
+	 * should be true if query or some subquery uses any session variable.
+	 */
+	pull_up_has_session_variables(root);
+
 	/*
 	 * If this is a simple UNION ALL query, flatten it into an appendrel. We
 	 * do this now because it requires applying pull_up_subqueries to the leaf
diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c
index a7b11b7f03..7486d975c7 100644
--- a/src/backend/optimizer/plan/setrefs.c
+++ b/src/backend/optimizer/plan/setrefs.c
@@ -172,6 +172,8 @@ static List *set_returning_clause_references(PlannerInfo *root,
 											 Plan *topplan,
 											 Index resultRelation,
 											 int rtoffset);
+static bool pull_up_has_session_variables_walker(Node *node,
+											   PlannerInfo *root);
 
 
 /*****************************************************************************
@@ -1127,6 +1129,50 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset)
 	return plan;
 }
 
+/*
+ * Search usage of session variables in subqueries
+ */
+void
+pull_up_has_session_variables(PlannerInfo *root)
+{
+	Query	   *query = root->parse;
+
+	if (query->hasSessionVariables)
+	{
+		root->hasSessionVariables = true;
+	}
+	else
+	{
+		(void) query_tree_walker(query,
+								 pull_up_has_session_variables_walker,
+								 (void *) root, 0);
+	}
+}
+
+static bool
+pull_up_has_session_variables_walker(Node *node, PlannerInfo *root)
+{
+	if (node == NULL)
+		return false;
+	if (IsA(node, Query))
+	{
+		Query	   *query = (Query *) node;
+
+		if (query->hasSessionVariables)
+		{
+			root->hasSessionVariables = true;
+			return false;
+		}
+
+		/* Recurse into subselects */
+		return query_tree_walker((Query *) node,
+								 pull_up_has_session_variables_walker,
+								 (void *) root, 0);
+	}
+	return expression_tree_walker(node, pull_up_has_session_variables_walker,
+								  (void *) root);
+}
+
 /*
  * set_indexonlyscan_references
  *		Do set_plan_references processing on an IndexOnlyScan
@@ -1794,15 +1840,39 @@ fix_expr_common(PlannerInfo *root, Node *node)
 				g->cols = cols;
 		}
 	}
+	else if (IsA(node, Param))
+	{
+		Param *p = (Param *) node;
+
+		if (p->paramkind == PARAM_VARIABLE)
+		{
+			PlanInvalItem *inval_item = makeNode(PlanInvalItem);
+
+			/* paramid is still session variable id */
+			inval_item->cacheId = VARIABLEOID;
+			inval_item->hashValue = GetSysCacheHashValue1(VARIABLEOID,
+														  ObjectIdGetDatum(p->paramvarid));
+
+			/* Append this variable to global, register dependency */
+			root->glob->invalItems = lappend(root->glob->invalItems,
+											 inval_item);
+		}
+	}
 }
 
 /*
  * fix_param_node
  *		Do set_plan_references processing on a Param
+ *		Collect session variables list and replace variable oid by
+ *		index to collected list.
  *
  * If it's a PARAM_MULTIEXPR, replace it with the appropriate Param from
  * root->multiexpr_params; otherwise no change is needed.
  * Just for paranoia's sake, we make a copy of the node in either case.
+ *
+ * If it's a PARAM_VARIABLE, then we collect used session variables in
+ * list root->glob->sessionVariable. We should to assign Param paramvarid
+ * too, and it is position of related session variable in mentioned list.
  */
 static Node *
 fix_param_node(PlannerInfo *root, Param *p)
@@ -1821,6 +1891,41 @@ fix_param_node(PlannerInfo *root, Param *p)
 			elog(ERROR, "unexpected PARAM_MULTIEXPR ID: %d", p->paramid);
 		return copyObject(list_nth(params, colno - 1));
 	}
+
+	if (p->paramkind == PARAM_VARIABLE)
+	{
+		ListCell   *lc;
+		int			n = 0;
+		bool		found = false;
+
+		/* We will modify object */
+		p = (Param *) copyObject(p);
+
+		/*
+		 * Now, we can actualize list of session variables, and we can complete
+		 * paramid parameter.
+		 */
+		foreach(lc, root->glob->sessionVariables)
+		{
+			if (lfirst_oid(lc) == p->paramvarid)
+			{
+				p->paramid = n;
+				found = true;
+				break;
+			}
+			n += 1;
+		}
+
+		if (!found)
+		{
+			root->glob->sessionVariables = lappend_oid(root->glob->sessionVariables,
+													  p->paramvarid);
+			p->paramid = n;
+		}
+
+		return (Node *) p;
+	}
+
 	return (Node *) copyObject(p);
 }
 
@@ -1882,7 +1987,10 @@ fix_alternative_subplan(PlannerInfo *root, AlternativeSubPlan *asplan,
  * replacing Aggref nodes that should be replaced by initplan output Params,
  * choosing the best implementation for AlternativeSubPlans,
  * looking up operator opcode info for OpExpr and related nodes,
- * and adding OIDs from regclass Const nodes into root->glob->relationOids.
+ * adding OIDs from regclass Const nodes into root->glob->relationOids,
+ * and assigning paramvarid to PARAM_VARIABLE params, and collecting
+ * of OIDs of session variables in root->glob->sessionVariables list
+ * (paramvarid is an possition of related session variable in this list).
  *
  * 'node': the expression to be modified
  * 'rtoffset': how much to increment varnos by
@@ -1904,7 +2012,8 @@ fix_scan_expr(PlannerInfo *root, Node *node, int rtoffset, double num_exec)
 		root->multiexpr_params != NIL ||
 		root->glob->lastPHId != 0 ||
 		root->minmax_aggs != NIL ||
-		root->hasAlternativeSubPlans)
+		root->hasAlternativeSubPlans ||
+		root->hasSessionVariables)
 	{
 		return fix_scan_expr_mutator(node, &context);
 	}
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index a707dc9f26..bd0470bffb 100644
--- a/src/backend/optimizer/util/clauses.c
+++ b/src/backend/optimizer/util/clauses.c
@@ -26,6 +26,7 @@
 #include "catalog/pg_operator.h"
 #include "catalog/pg_proc.h"
 #include "catalog/pg_type.h"
+#include "commands/session_variable.h"
 #include "executor/executor.h"
 #include "executor/functions.h"
 #include "funcapi.h"
@@ -819,7 +820,8 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 	{
 		Param	   *param = (Param *) node;
 
-		if (param->paramkind == PARAM_EXTERN)
+		if (param->paramkind == PARAM_EXTERN ||
+			param->paramkind == PARAM_VARIABLE)
 			return false;
 
 		if (param->paramkind != PARAM_EXEC ||
@@ -2229,6 +2231,7 @@ convert_saop_to_hashed_saop_walker(Node *node, void *context)
  *	  value of the Param.
  * 2. Fold stable, as well as immutable, functions to constants.
  * 3. Reduce PlaceHolderVar nodes to their contained expressions.
+ * 4. Current value of session variable can be used for estimation too.
  *--------------------
  */
 Node *
@@ -2351,6 +2354,30 @@ eval_const_expressions_mutator(Node *node,
 						}
 					}
 				}
+				else if (param->paramkind == PARAM_VARIABLE &&
+						 context->estimate)
+				{
+					int16		typLen;
+					bool		typByVal;
+					Datum		pval;
+					bool		isnull;
+
+					get_typlenbyval(param->paramtype,
+									&typLen, &typByVal);
+
+					pval = GetSessionVariable(param->paramvarid,
+											 &isnull,
+											 param->paramtype,
+											 true);
+
+					return (Node *) makeConst(param->paramtype,
+											  param->paramtypmod,
+											  param->paramcollid,
+											  (int) typLen,
+											  pval,
+											  isnull,
+											  typByVal);
+				}
 
 				/*
 				 * Not replaceable, so just copy the Param (no need to
@@ -4735,22 +4762,46 @@ substitute_actual_parameters_mutator(Node *node,
 {
 	if (node == NULL)
 		return NULL;
+
+	/*
+	 * The expression of SQL function can contain params of two separated
+	 * by different paramkind field. The nodes with paramkind PARAM_EXTERN
+	 * are related to function's arguments (and should be replaced in this
+	 * step), because this is mechanism how we apply the function's arguments
+	 * for an expression.
+	 *
+	 * The nodes with paramkind PARAM_VARIABLE are related to an usage of
+	 * session variables. The values of session variables are not passed
+	 * to expression by expression arguments, so it should not be replaced
+	 * here by function's arguments. Although we can substitute params
+	 * related to immutable session variables with default expression by
+	 * this default expression, it is safer don't do it. We don't need to
+	 * run security checks here. There can be some performance loss, but
+	 * an access to session variable is fast (and the result of default
+	 * expression is immediately materialized and can be reused).
+	 */
 	if (IsA(node, Param))
 	{
 		Param	   *param = (Param *) node;
 
-		if (param->paramkind != PARAM_EXTERN)
+		if (param->paramkind != PARAM_EXTERN &&
+			  param->paramkind != PARAM_VARIABLE)
 			elog(ERROR, "unexpected paramkind: %d", (int) param->paramkind);
-		if (param->paramid <= 0 || param->paramid > context->nargs)
-			elog(ERROR, "invalid paramid: %d", param->paramid);
 
-		/* Count usage of parameter */
-		context->usecounts[param->paramid - 1]++;
+		if (param->paramkind == PARAM_EXTERN)
+		{
+			if (param->paramid <= 0 || param->paramid > context->nargs)
+				elog(ERROR, "invalid paramid: %d", param->paramid);
 
-		/* Select the appropriate actual arg and replace the Param with it */
-		/* We don't need to copy at this time (it'll get done later) */
-		return list_nth(context->args, param->paramid - 1);
+			/* Count usage of parameter */
+			context->usecounts[param->paramid - 1]++;
+
+			/* Select the appropriate actual arg and replace the Param with it */
+			/* We don't need to copy at this time (it'll get done later) */
+			return list_nth(context->args, param->paramid - 1);
+		}
 	}
+
 	return expression_tree_mutator(node, substitute_actual_parameters_mutator,
 								   (void *) context);
 }
diff --git a/src/backend/parser/analyze.c b/src/backend/parser/analyze.c
index 6ac2e9ce23..fc91412566 100644
--- a/src/backend/parser/analyze.c
+++ b/src/backend/parser/analyze.c
@@ -25,8 +25,11 @@
 #include "postgres.h"
 
 #include "access/sysattr.h"
+#include "catalog/namespace.h"
 #include "catalog/pg_proc.h"
 #include "catalog/pg_type.h"
+#include "catalog/pg_variable.h"
+#include "commands/session_variable.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
@@ -50,6 +53,7 @@
 #include "utils/builtins.h"
 #include "utils/guc.h"
 #include "utils/queryjumble.h"
+#include "utils/lsyscache.h"
 #include "utils/rel.h"
 #include "utils/syscache.h"
 
@@ -88,6 +92,8 @@ static Query *transformCreateTableAsStmt(ParseState *pstate,
 										 CreateTableAsStmt *stmt);
 static Query *transformCallStmt(ParseState *pstate,
 								CallStmt *stmt);
+static Query *transformLetStmt(ParseState *pstate,
+							   LetStmt * stmt);
 static void transformLockingClause(ParseState *pstate, Query *qry,
 								   LockingClause *lc, bool pushedDown);
 #ifdef RAW_EXPRESSION_COVERAGE_TEST
@@ -289,6 +295,7 @@ transformStmt(ParseState *pstate, Node *parseTree)
 		case T_InsertStmt:
 		case T_UpdateStmt:
 		case T_DeleteStmt:
+		case T_LetStmt:
 			(void) test_raw_expression_coverage(parseTree, NULL);
 			break;
 		default:
@@ -358,6 +365,11 @@ transformStmt(ParseState *pstate, Node *parseTree)
 									   (CallStmt *) parseTree);
 			break;
 
+		case T_LetStmt:
+			result = transformLetStmt(pstate,
+									  (LetStmt *) parseTree);
+			break;
+
 		default:
 
 			/*
@@ -399,6 +411,7 @@ analyze_requires_snapshot(RawStmt *parseTree)
 		case T_UpdateStmt:
 		case T_SelectStmt:
 		case T_PLAssignStmt:
+		case T_LetStmt:
 			result = true;
 			break;
 
@@ -482,6 +495,8 @@ transformDeleteStmt(ParseState *pstate, DeleteStmt *stmt)
 	qry->hasTargetSRFs = pstate->p_hasTargetSRFs;
 	qry->hasAggs = pstate->p_hasAggs;
 
+	qry->hasSessionVariables = pstate->p_hasSessionVariables;
+
 	assign_query_collations(pstate, qry);
 
 	/* this must be done after collations, for reliable comparison of exprs */
@@ -899,6 +914,7 @@ transformInsertStmt(ParseState *pstate, InsertStmt *stmt)
 
 	qry->hasTargetSRFs = pstate->p_hasTargetSRFs;
 	qry->hasSubLinks = pstate->p_hasSubLinks;
+	qry->hasSessionVariables = pstate->p_hasSessionVariables;
 
 	assign_query_collations(pstate, qry);
 
@@ -1269,8 +1285,9 @@ transformSelectStmt(ParseState *pstate, SelectStmt *stmt)
 	/* process the FROM clause */
 	transformFromClause(pstate, stmt->fromClause);
 
-	/* transform targetlist */
-	qry->targetList = transformTargetList(pstate, stmt->targetList,
+	/* Transform targetlist. */
+	qry->targetList = transformTargetList(pstate,
+										  stmt->targetList,
 										  EXPR_KIND_SELECT_TARGET);
 
 	/* mark column origins */
@@ -1361,6 +1378,8 @@ transformSelectStmt(ParseState *pstate, SelectStmt *stmt)
 							   (LockingClause *) lfirst(l), false);
 	}
 
+	qry->hasSessionVariables = pstate->p_hasSessionVariables;
+
 	assign_query_collations(pstate, qry);
 
 	/* this must be done after collations, for reliable comparison of exprs */
@@ -1585,12 +1604,268 @@ transformValuesClause(ParseState *pstate, SelectStmt *stmt)
 	qry->jointree = makeFromExpr(pstate->p_joinlist, NULL);
 
 	qry->hasSubLinks = pstate->p_hasSubLinks;
+	qry->hasSessionVariables = pstate->p_hasSessionVariables;
 
 	assign_query_collations(pstate, qry);
 
 	return qry;
 }
 
+/*
+ * transformLetStmt -
+ *	  transform an Let Statement
+ */
+static Query *
+transformLetStmt(ParseState *pstate, LetStmt * stmt)
+{
+	Query	   *query;
+	Query	   *result;
+	List	   *exprList = NIL;
+	List	   *exprListCoer = NIL;
+	List	   *indirection = NIL;
+	ListCell   *lc;
+	Query	   *selectQuery;
+	int			i = 0;
+	Oid			varid;
+	char	   *attrname = NULL;
+	bool		not_unique;
+	bool		is_rowtype;
+	Oid			typid;
+	int32		typmod;
+	Oid			collid;
+	AclResult	aclresult;
+	List	   *names = NULL;
+	int			indirection_start;
+
+	/* There can't be any outer WITH to worry about */
+	Assert(pstate->p_ctenamespace == NIL);
+
+	names = NamesFromList(stmt->target);
+
+	varid = IdentifyVariable(names, &attrname, true, &not_unique);
+	if (not_unique)
+		ereport(ERROR,
+				(errcode(ERRCODE_AMBIGUOUS_PARAMETER),
+				 errmsg("target \"%s\" of LET command is ambiguous",
+						NameListToString(names)),
+				 parser_errposition(pstate, stmt->location)));
+
+	if (!OidIsValid(varid))
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_OBJECT),
+				 errmsg("session variable \"%s\" doesn't exist",
+						NameListToString(names)),
+				 parser_errposition(pstate, stmt->location)));
+
+	indirection_start = list_length(names) - (attrname ? 1 : 0);
+	indirection = list_copy_tail(stmt->target, indirection_start);
+
+	get_session_variable_type_typmod_collid(varid, &typid, &typmod, &collid);
+
+	is_rowtype = type_is_rowtype(typid);
+
+	if (attrname && !is_rowtype)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("target variable \"%s\" is not row type",
+						session_variable_get_name(varid)),
+				 parser_errposition(pstate, stmt->location)));
+
+	if (stmt->set_default && attrname != NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("only complete variable can be set to default"),
+				 parser_errposition(pstate, stmt->location)));
+
+	aclresult = pg_variable_aclcheck(varid, GetUserId(), ACL_WRITE);
+	if (aclresult != ACLCHECK_OK)
+		aclcheck_error(aclresult, OBJECT_VARIABLE, NameListToString(names));
+
+	pstate->p_expr_kind = EXPR_KIND_LET_TARGET;
+
+	/*
+	 * stmt->query is SelectStmt node. An tranformation of
+	 * this node doesn't support SetToDefault node. Instead injecting
+	 * of transformSelectStmt or parse state, we can directly
+	 * transform target list here if holds SetToDefault node.
+	 */
+	if (stmt->set_default)
+	{
+		selectQuery = makeNode(Query);
+		selectQuery->commandType = CMD_SELECT;
+
+		/*
+		 * ResTarget(SetToDefault) -> TargetEntry(expr(SetToDefault)) */
+		selectQuery->targetList = transformTargetList(pstate,
+													  ((SelectStmt *) stmt->query)->targetList,
+													  EXPR_KIND_LET_TARGET);
+	}
+	else
+		selectQuery = transformStmt(pstate, stmt->query);
+
+	/* The grammar should have produced a SELECT */
+	if (!IsA(selectQuery, Query) ||
+		selectQuery->commandType != CMD_SELECT)
+		elog(ERROR, "unexpected non-SELECT command in LET command");
+
+	/*----------
+	 * Generate an expression list for the LET that selects all the
+	 * non-resjunk columns from the subquery.
+	 *----------
+	 */
+	exprList = NIL;
+	foreach(lc, selectQuery->targetList)
+	{
+		TargetEntry *tle = (TargetEntry *) lfirst(lc);
+
+		if (tle->resjunk)
+			continue;
+
+		exprList = lappend(exprList, tle->expr);
+	}
+
+	/* don't allow multicolumn result */
+	if (list_length(exprList) != 1)
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 errmsg("expression is not scalar value"),
+				 parser_errposition(pstate,
+									exprLocation((Node *) exprList))));
+
+	exprListCoer = NIL;
+
+	foreach(lc, exprList)
+	{
+		Expr	   *expr = (Expr *) lfirst(lc);
+		Expr	   *coerced_expr;
+		Param	   *param;
+		Oid			exprtypid;
+
+		if (IsA(expr, SetToDefault))
+		{
+			SetToDefault *def = (SetToDefault *) expr;
+
+			def->typeId = typid;
+			def->typeMod = typmod;
+			def->collation = collid;
+		}
+		else if (IsA(expr, Const) && ((Const *) expr)->constisnull)
+		{
+			/* use known type for NULL value */
+			expr = (Expr *) makeNullConst(typid, typmod, collid);
+		}
+
+		/* now we can read type of expression */
+		exprtypid = exprType((Node *) expr);
+
+		param = makeNode(Param);
+		param->paramkind = PARAM_VARIABLE;
+		param->paramvarid = varid;
+		param->paramtype = typid;
+		param->paramtypmod = typmod;
+
+		if (indirection != NULL)
+		{
+			bool		targetIsArray;
+			char	   *targetName;
+
+			targetName = attrname != NULL ? attrname : get_session_variable_name(varid);
+			targetIsArray = OidIsValid(get_element_type(typid));
+
+			pstate->p_hasSessionVariables = true;
+
+			coerced_expr = (Expr *)
+				transformAssignmentIndirection(pstate,
+											   (Node *) param,
+											   targetName,
+											   targetIsArray,
+											   typid,
+											   typmod,
+											   InvalidOid,
+											   indirection,
+											   list_head(indirection),
+											   (Node *) expr,
+											   COERCION_PLPGSQL,
+											   stmt->location);
+		}
+		else
+			coerced_expr = (Expr *)
+				coerce_to_target_type(pstate,
+									  (Node *) expr,
+									  exprtypid,
+									  typid, typmod,
+									  COERCION_ASSIGNMENT,
+									  COERCE_IMPLICIT_CAST,
+									  stmt->location);
+
+		if (coerced_expr == NULL)
+			ereport(ERROR,
+					(errcode(ERRCODE_DATATYPE_MISMATCH),
+					 errmsg("variable \"%s\" is of type %s,"
+							" but expression is of type %s",
+							session_variable_get_name(varid),
+							format_type_be(typid),
+							format_type_be(exprtypid)),
+					 errhint("You will need to rewrite or cast the expression."),
+					 parser_errposition(pstate, exprLocation((Node *) expr))));
+
+		exprListCoer = lappend(exprListCoer, coerced_expr);
+	}
+
+	/*
+	 * Generate query's target list using the computed list of expressions.
+	 */
+	query = makeNode(Query);
+	query->commandType = CMD_SELECT;
+
+	foreach(lc, exprListCoer)
+	{
+		Expr	   *expr = (Expr *) lfirst(lc);
+		TargetEntry *tle;
+
+		tle = makeTargetEntry(expr,
+							  i + 1,
+							  FigureColname((Node *) expr),
+							  false);
+		query->targetList = lappend(query->targetList, tle);
+	}
+
+	/* done building the range table and jointree */
+	query->rtable = pstate->p_rtable;
+	query->jointree = makeFromExpr(pstate->p_joinlist, NULL);
+
+	query->hasTargetSRFs = pstate->p_hasTargetSRFs;
+	query->hasSubLinks = pstate->p_hasSubLinks;
+	query->hasSessionVariables = pstate->p_hasSessionVariables;
+
+	/* This is top query */
+	query->canSetTag = true;
+
+	/*
+	 * rewrite SetToDefaults needs varid in Query structure
+	 */
+	query->resultVariable = varid;
+
+	assign_query_collations(pstate, query);
+
+	stmt->query = (Node *) query;
+
+	/*
+	 * When statement is executed as an expression as PLpgSQL LET
+	 * statement, then we should return query. We should not
+	 * use a utility wrapper node.
+	 */
+	if (stmt->plpgsql_mode)
+		return query;
+
+	/* represent the command as a utility Query */
+	result = makeNode(Query);
+	result->commandType = CMD_UTILITY;
+	result->utilityStmt = (Node *) stmt;
+
+	return result;
+}
+
 /*
  * transformSetOperationStmt -
  *	  transforms a set-operations tree
@@ -1841,6 +2116,8 @@ transformSetOperationStmt(ParseState *pstate, SelectStmt *stmt)
 							   (LockingClause *) lfirst(l), false);
 	}
 
+	qry->hasSessionVariables = pstate->p_hasSessionVariables;
+
 	assign_query_collations(pstate, qry);
 
 	/* this must be done after collations, for reliable comparison of exprs */
@@ -2369,6 +2646,7 @@ transformUpdateStmt(ParseState *pstate, UpdateStmt *stmt)
 
 	qry->hasTargetSRFs = pstate->p_hasTargetSRFs;
 	qry->hasSubLinks = pstate->p_hasSubLinks;
+	qry->hasSessionVariables = pstate->p_hasSessionVariables;
 
 	assign_query_collations(pstate, qry);
 
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index b5966712ce..f26cbcde30 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -244,6 +244,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	JoinType			jtype;
 	DropBehavior		dbehavior;
 	OnCommitAction		oncommit;
+	VariableEOXAction	oneoxaction;
 	List				*list;
 	Node				*node;
 	ObjectType			objtype;
@@ -296,8 +297,8 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 		ConstraintsSetStmt CopyStmt CreateAsStmt CreateCastStmt
 		CreateDomainStmt CreateExtensionStmt CreateGroupStmt CreateOpClassStmt
 		CreateOpFamilyStmt AlterOpFamilyStmt CreatePLangStmt
-		CreateSchemaStmt CreateSeqStmt CreateStmt CreateStatsStmt CreateTableSpaceStmt
-		CreateFdwStmt CreateForeignServerStmt CreateForeignTableStmt
+		CreateSchemaStmt CreateSessionVarStmt CreateSeqStmt CreateStmt CreateStatsStmt
+		CreateTableSpaceStmt CreateFdwStmt CreateForeignServerStmt CreateForeignTableStmt
 		CreateAssertionStmt CreateTransformStmt CreateTrigStmt CreateEventTrigStmt
 		CreateUserStmt CreateUserMappingStmt CreateRoleStmt CreatePolicyStmt
 		CreatedbStmt DeclareCursorStmt DefineStmt DeleteStmt DiscardStmt DoStmt
@@ -307,7 +308,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 		DropTransformStmt
 		DropUserMappingStmt ExplainStmt FetchStmt
 		GrantStmt GrantRoleStmt ImportForeignSchemaStmt IndexStmt InsertStmt
-		ListenStmt LoadStmt LockStmt NotifyStmt ExplainableStmt PreparableStmt
+		LetStmt ListenStmt LoadStmt LockStmt NotifyStmt ExplainableStmt PreparableStmt
 		CreateFunctionStmt AlterFunctionStmt ReindexStmt RemoveAggrStmt
 		RemoveFuncStmt RemoveOperStmt RenameStmt ReturnStmt RevokeStmt RevokeRoleStmt
 		RuleActionStmt RuleActionStmtOrEmpty RuleStmt
@@ -447,6 +448,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 				TriggerTransitions TriggerReferencing
 				vacuum_relation_list opt_vacuum_relation_list
 				drop_option_list pub_obj_list
+				let_target
 
 %type <node>	opt_routine_body
 %type <groupclause> group_clause
@@ -469,6 +471,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %type <ival>	 OptTemp
 %type <ival>	 OptNoLog
 %type <oncommit> OnCommitOption
+%type <oneoxaction> OnEOXActionOption
 
 %type <ival>	for_locking_strength
 %type <node>	for_locking_item
@@ -632,6 +635,8 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %type <partboundspec> PartitionBoundSpec
 %type <list>		hash_partbound
 %type <defelt>		hash_partbound_elem
+%type <node>		OptSessionVarDefExpr
+%type <boolean>		OptNotNull OptImmutable
 
 
 /*
@@ -701,7 +706,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	KEY
 
 	LABEL LANGUAGE LARGE_P LAST_P LATERAL_P
-	LEADING LEAKPROOF LEAST LEFT LEVEL LIKE LIMIT LISTEN LOAD LOCAL
+	LEADING LEAKPROOF LEAST LEFT LET LEVEL LIKE LIMIT LISTEN LOAD LOCAL
 	LOCALTIME LOCALTIMESTAMP LOCATION LOCK_P LOCKED LOGGED
 
 	MAPPING MATCH MATERIALIZED MAXVALUE METHOD MINUTE_P MINVALUE MODE MONTH_P MOVE
@@ -740,8 +745,8 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 	UESCAPE UNBOUNDED UNCOMMITTED UNENCRYPTED UNION UNIQUE UNKNOWN
 	UNLISTEN UNLOGGED UNTIL UPDATE USER USING
 
-	VACUUM VALID VALIDATE VALIDATOR VALUE_P VALUES VARCHAR VARIADIC VARYING
-	VERBOSE VERSION_P VIEW VIEWS VOLATILE
+	VACUUM VALID VALIDATE VALIDATOR VALUE_P VALUES VARCHAR VARIABLE VARIABLES
+	VARIADIC VARYING VERBOSE VERSION_P VIEW VIEWS VOLATILE
 
 	WHEN WHERE WHITESPACE_P WINDOW WITH WITHIN WITHOUT WORK WRAPPER WRITE
 
@@ -776,6 +781,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %token		MODE_PLPGSQL_ASSIGN1
 %token		MODE_PLPGSQL_ASSIGN2
 %token		MODE_PLPGSQL_ASSIGN3
+%token		MODE_PLPGSQL_LET
 
 
 /* Precedence: lowest to highest */
@@ -881,6 +887,13 @@ parse_toplevel:
 				pg_yyget_extra(yyscanner)->parsetree =
 					list_make1(makeRawStmt((Node *) n, 0));
 			}
+			| MODE_PLPGSQL_LET LetStmt
+			{
+				LetStmt *n = (LetStmt *) $2;
+				n->plpgsql_mode = true;
+				pg_yyget_extra(yyscanner)->parsetree =
+					list_make1(makeRawStmt((Node *) n, 0));
+			}
 		;
 
 /*
@@ -984,6 +997,7 @@ stmt:
 			| CreatePolicyStmt
 			| CreatePLangStmt
 			| CreateSchemaStmt
+			| CreateSessionVarStmt
 			| CreateSeqStmt
 			| CreateStmt
 			| CreateSubscriptionStmt
@@ -1021,6 +1035,7 @@ stmt:
 			| ImportForeignSchemaStmt
 			| IndexStmt
 			| InsertStmt
+			| LetStmt
 			| ListenStmt
 			| RefreshMatViewStmt
 			| LoadStmt
@@ -1913,7 +1928,12 @@ DiscardStmt:
 					n->target = DISCARD_SEQUENCES;
 					$$ = (Node *) n;
 				}
-
+			| DISCARD VARIABLES
+				{
+					DiscardStmt *n = makeNode(DiscardStmt);
+					n->target = DISCARD_VARIABLES;
+					$$ = (Node *) n;
+				}
 		;
 
 
@@ -4743,6 +4763,69 @@ create_extension_opt_item:
 				}
 		;
 
+/*****************************************************************************
+ *
+ *		QUERY :
+ *				CREATE VARIABLE varname [AS] type
+ *
+ *****************************************************************************/
+
+CreateSessionVarStmt:
+			CREATE OptTemp OptImmutable VARIABLE qualified_name opt_as Typename opt_collate_clause OptNotNull OptSessionVarDefExpr OnEOXActionOption
+				{
+					CreateSessionVarStmt *n = makeNode(CreateSessionVarStmt);
+					$5->relpersistence = $2;
+					n->is_immutable = $3;
+					n->variable = $5;
+					n->typeName = $7;
+					n->collClause = (CollateClause *) $8;
+					n->is_not_null = $9;
+					n->defexpr = $10;
+					n->eoxaction = $11;
+					n->if_not_exists = false;
+					$$ = (Node *) n;
+				}
+			| CREATE OptTemp OptImmutable VARIABLE IF_P NOT EXISTS qualified_name opt_as Typename opt_collate_clause OptNotNull OptSessionVarDefExpr OnEOXActionOption
+				{
+					CreateSessionVarStmt *n = makeNode(CreateSessionVarStmt);
+					$8->relpersistence = $2;
+					n->is_immutable = $3;
+					n->variable = $8;
+					n->typeName = $10;
+					n->collClause = (CollateClause *) $11;
+					n->is_not_null = $12;
+					n->defexpr = $13;
+					n->eoxaction = $14;
+					n->if_not_exists = true;
+					$$ = (Node *) n;
+				}
+		;
+
+OptSessionVarDefExpr: DEFAULT b_expr					{ $$ = $2; }
+			| /* EMPTY */							{ $$ = NULL; }
+		;
+
+/*
+ * Temporary session variables can be dropped on successful
+ * transaction end like tables. We can force only reset on
+ * transaction end. Because the session variables are not
+ * transactional, we have calculate with ROLLBACK too.
+ * The clause ON TRANSACTION END is more illustrative
+ * synonymum to ON COMMIT ROLLBACK RESET.
+ */
+OnEOXActionOption:  ON COMMIT DROP					{ $$ = VARIABLE_EOX_DROP; }
+			| ON TRANSACTION END_P RESET			{ $$ = VARIABLE_EOX_RESET; }
+			| /*EMPTY*/								{ $$ = VARIABLE_EOX_NOOP; }
+		;
+
+OptNotNull: NOT NULL_P								{ $$ = true; }
+			| /* EMPTY */							{ $$ = false; }
+		;
+
+OptImmutable: IMMUTABLE								{ $$ = true; }
+			| /* EMPTY */							{ $$ = false; }
+		;
+
 /*****************************************************************************
  *
  * ALTER EXTENSION name UPDATE [ TO version ]
@@ -6430,6 +6513,7 @@ object_type_any_name:
 			| TEXT_P SEARCH DICTIONARY				{ $$ = OBJECT_TSDICTIONARY; }
 			| TEXT_P SEARCH TEMPLATE				{ $$ = OBJECT_TSTEMPLATE; }
 			| TEXT_P SEARCH CONFIGURATION			{ $$ = OBJECT_TSCONFIGURATION; }
+			| VARIABLE								{ $$ = OBJECT_VARIABLE; }
 		;
 
 /*
@@ -7198,6 +7282,14 @@ privilege_target:
 					n->objs = $2;
 					$$ = n;
 				}
+			| VARIABLE qualified_name_list
+				{
+					PrivTarget *n = (PrivTarget *) palloc(sizeof(PrivTarget));
+					n->targtype = ACL_TARGET_OBJECT;
+					n->objtype = OBJECT_VARIABLE;
+					n->objs = $2;
+					$$ = n;
+				}
 			| ALL TABLES IN_P SCHEMA name_list
 				{
 					PrivTarget *n = (PrivTarget *) palloc(sizeof(PrivTarget));
@@ -7238,6 +7330,14 @@ privilege_target:
 					n->objs = $5;
 					$$ = n;
 				}
+			| ALL VARIABLES IN_P SCHEMA name_list
+				{
+					PrivTarget *n = (PrivTarget *) palloc(sizeof(PrivTarget));
+					n->targtype = ACL_TARGET_ALL_IN_SCHEMA;
+					n->objtype = OBJECT_VARIABLE;
+					n->objs = $5;
+					$$ = n;
+				}
 		;
 
 
@@ -7398,6 +7498,7 @@ defacl_privilege_target:
 			| SEQUENCES		{ $$ = OBJECT_SEQUENCE; }
 			| TYPES_P		{ $$ = OBJECT_TYPE; }
 			| SCHEMAS		{ $$ = OBJECT_SCHEMA; }
+			| VARIABLES		{ $$ = OBJECT_VARIABLE; }
 		;
 
 
@@ -9100,6 +9201,25 @@ RenameStmt: ALTER AGGREGATE aggregate_with_argtypes RENAME TO name
 					n->missing_ok = false;
 					$$ = (Node *)n;
 				}
+			| ALTER VARIABLE any_name RENAME TO name
+				{
+					RenameStmt *n = makeNode(RenameStmt);
+					n->renameType = OBJECT_VARIABLE;
+					n->object = (Node *) $3;
+					n->newname = $6;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			| ALTER VARIABLE IF_P EXISTS any_name RENAME TO name
+				{
+					RenameStmt *n = makeNode(RenameStmt);
+					n->renameType = OBJECT_VARIABLE;
+					n->object = (Node *) $5;
+					n->newname = $8;
+					n->missing_ok = true;
+					$$ = (Node *)n;
+				}
+
 		;
 
 opt_column: COLUMN
@@ -9428,6 +9548,25 @@ AlterObjectSchemaStmt:
 					n->missing_ok = false;
 					$$ = (Node *)n;
 				}
+			| ALTER VARIABLE any_name SET SCHEMA name
+				{
+					AlterObjectSchemaStmt *n = makeNode(AlterObjectSchemaStmt);
+					n->objectType = OBJECT_VARIABLE;
+					n->object = (Node *) $3;
+					n->newschema = $6;
+					n->missing_ok = false;
+					$$ = (Node *)n;
+				}
+			| ALTER VARIABLE IF_P EXISTS any_name SET SCHEMA name
+				{
+					AlterObjectSchemaStmt *n = makeNode(AlterObjectSchemaStmt);
+					n->objectType = OBJECT_VARIABLE;
+					n->object = (Node *) $5;
+					n->newschema = $8;
+					n->missing_ok = true;
+					$$ = (Node *)n;
+				}
+
 		;
 
 /*****************************************************************************
@@ -9681,6 +9820,14 @@ AlterOwnerStmt: ALTER AGGREGATE aggregate_with_argtypes OWNER TO RoleSpec
 					n->newowner = $6;
 					$$ = (Node *)n;
 				}
+			| ALTER VARIABLE any_name OWNER TO RoleSpec
+				{
+					AlterOwnerStmt *n = makeNode(AlterOwnerStmt);
+					n->objectType = OBJECT_VARIABLE;
+					n->object = (Node *) $3;
+					n->newowner = $6;
+					$$ = (Node *)n;
+				}
 		;
 
 
@@ -11016,6 +11163,7 @@ ExplainableStmt:
 			| CreateMatViewStmt
 			| RefreshMatViewStmt
 			| ExecuteStmt					/* by default all are $$=$1 */
+			| LetStmt
 		;
 
 /*****************************************************************************
@@ -11044,6 +11192,7 @@ PreparableStmt:
 			| InsertStmt
 			| UpdateStmt
 			| DeleteStmt					/* by default all are $$=$1 */
+			| LetStmt
 		;
 
 /*****************************************************************************
@@ -11461,6 +11610,49 @@ opt_hold: /* EMPTY */						{ $$ = 0; }
 			| WITHOUT HOLD					{ $$ = 0; }
 		;
 
+/*****************************************************************************
+ *
+ *		QUERY:
+ *				LET STATEMENTS
+ *
+ *****************************************************************************/
+LetStmt:	LET let_target '=' a_expr
+				{
+					LetStmt	   *n = makeNode(LetStmt);
+					SelectStmt *select;
+					ResTarget  *res;
+
+					n->target = $2;
+
+					select = makeNode(SelectStmt);
+					res = makeNode(ResTarget);
+
+					/* Create target list for implicit query */
+					res->name = NULL;
+					res->indirection = NIL;
+					res->val = (Node *) $4;
+					res->location = @4;
+
+					select->targetList = list_make1(res);
+					n->query = (Node *) select;
+
+					n->set_default = IsA($4, SetToDefault);
+
+					n->location = @2;
+					$$ = (Node *) n;
+				}
+		;
+
+let_target:
+			ColId opt_indirection
+				{
+					$$ = list_make1(makeString($1));
+					if ($2)
+						  $$ = list_concat($$,
+										   check_indirection($2, yyscanner));
+				}
+		;
+
 /*****************************************************************************
  *
  *		QUERY:
@@ -15769,6 +15961,7 @@ unreserved_keyword:
 			| LARGE_P
 			| LAST_P
 			| LEAKPROOF
+			| LET
 			| LEVEL
 			| LISTEN
 			| LOAD
@@ -15926,6 +16119,8 @@ unreserved_keyword:
 			| VALIDATE
 			| VALIDATOR
 			| VALUE_P
+			| VARIABLE
+			| VARIABLES
 			| VARYING
 			| VERSION_P
 			| VIEW
@@ -16333,6 +16528,7 @@ bare_label_keyword:
 			| LEAKPROOF
 			| LEAST
 			| LEFT
+			| LET
 			| LEVEL
 			| LIKE
 			| LISTEN
@@ -16532,6 +16728,8 @@ bare_label_keyword:
 			| VALUE_P
 			| VALUES
 			| VARCHAR
+			| VARIABLE
+			| VARIABLES
 			| VARIADIC
 			| VERBOSE
 			| VERSION_P
diff --git a/src/backend/parser/parse_agg.c b/src/backend/parser/parse_agg.c
index ded0a14d72..280eb71b71 100644
--- a/src/backend/parser/parse_agg.c
+++ b/src/backend/parser/parse_agg.c
@@ -348,6 +348,7 @@ check_agglevels_and_constraints(ParseState *pstate, Node *expr)
 			Assert(false);		/* can't happen */
 			break;
 		case EXPR_KIND_OTHER:
+		case EXPR_KIND_LET_TARGET:
 
 			/*
 			 * Accept aggregate/grouping here; caller must throw error if
@@ -464,6 +465,7 @@ check_agglevels_and_constraints(ParseState *pstate, Node *expr)
 			break;
 		case EXPR_KIND_COLUMN_DEFAULT:
 		case EXPR_KIND_FUNCTION_DEFAULT:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 
 			if (isAgg)
 				err = _("aggregate functions are not allowed in DEFAULT expressions");
@@ -905,6 +907,7 @@ transformWindowFuncCall(ParseState *pstate, WindowFunc *wfunc,
 			break;
 		case EXPR_KIND_COLUMN_DEFAULT:
 		case EXPR_KIND_FUNCTION_DEFAULT:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 			err = _("window functions are not allowed in DEFAULT expressions");
 			break;
 		case EXPR_KIND_INDEX_EXPRESSION:
@@ -943,6 +946,9 @@ transformWindowFuncCall(ParseState *pstate, WindowFunc *wfunc,
 		case EXPR_KIND_CYCLE_MARK:
 			errkind = true;
 			break;
+		case EXPR_KIND_LET_TARGET:
+			err = _("window functions are not allowed in LET statement");
+			break;
 
 			/*
 			 * There is intentionally no default: case here, so that the
diff --git a/src/backend/parser/parse_expr.c b/src/backend/parser/parse_expr.c
index c4aaf37727..86b8eb2e25 100644
--- a/src/backend/parser/parse_expr.c
+++ b/src/backend/parser/parse_expr.c
@@ -16,6 +16,7 @@
 #include "postgres.h"
 
 #include "catalog/pg_type.h"
+#include "catalog/pg_variable.h"
 #include "commands/dbcommands.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
@@ -36,11 +37,12 @@
 #include "utils/date.h"
 #include "utils/lsyscache.h"
 #include "utils/timestamp.h"
+#include "utils/typcache.h"
 #include "utils/xml.h"
 
 /* GUC parameters */
 bool		Transform_null_equals = false;
-
+bool		session_variables_ambiguity_warning = false;
 
 static Node *transformExprRecurse(ParseState *pstate, Node *expr);
 static Node *transformParamRef(ParseState *pstate, ParamRef *pref);
@@ -82,6 +84,9 @@ static Expr *make_distinct_op(ParseState *pstate, List *opname,
 							  Node *ltree, Node *rtree, int location);
 static Node *make_nulltest_from_distinct(ParseState *pstate,
 										 A_Expr *distincta, Node *arg);
+static Node *makeParamSessionVariable(ParseState *pstate,
+									 Oid varid, Oid typid, int32 typmod, Oid collid,
+									 char *attrname, int location);
 
 
 /*
@@ -443,6 +448,10 @@ transformColumnRef(ParseState *pstate, ColumnRef *cref)
 	char	   *relname = NULL;
 	char	   *colname = NULL;
 	ParseNamespaceItem *nsitem;
+	Oid			varid = InvalidOid;
+	char	   *attrname = NULL;
+	bool		not_unique;
+	bool		lockit;
 	int			levels_up;
 	enum
 	{
@@ -504,6 +513,9 @@ transformColumnRef(ParseState *pstate, ColumnRef *cref)
 		case EXPR_KIND_COPY_WHERE:
 		case EXPR_KIND_GENERATED_COLUMN:
 		case EXPR_KIND_CYCLE_MARK:
+		case EXPR_KIND_VARIABLE_DEFAULT:
+		case EXPR_KIND_LET_TARGET:
+
 			/* okay */
 			break;
 
@@ -785,6 +797,123 @@ transformColumnRef(ParseState *pstate, ColumnRef *cref)
 					 parser_errposition(pstate, cref->location)));
 	}
 
+	/* Protect session variable by AccessShareLock when it is not shadowed */
+	lockit = (node == NULL);
+
+	/* The col's reference can be reference to session variable too. */
+	varid = IdentifyVariable(cref->fields, &attrname, lockit, &not_unique);
+
+	/*
+	 * Raise error when reference to session variable is ambiguous and
+	 * and this reference is not shadowed.
+	 */
+	if (!node && not_unique)
+		ereport(ERROR,
+				(errcode(ERRCODE_AMBIGUOUS_PARAMETER),
+				 errmsg("session variable reference \"%s\" is ambiguous",
+						NameListToString(cref->fields)),
+				 parser_errposition(pstate, cref->location)));
+
+	if (OidIsValid(varid))
+	{
+		/*
+		 * When Session variables is shadowed by columns or by routine's
+		 * variables or routine's arguments.
+		 */
+		if (node != NULL)
+		{
+			/*
+			 * In this case we can raise warning (when it is required).
+			 * These warnings can be reduced. We should not to raise
+			 * warning for contexts where usage of session variables
+			 * has not sense. We should not to raise warning when we
+			 * can detect so variable has not wanted field (and then
+			 * there is not possible identifier's collision).
+			 */
+			if (session_variables_ambiguity_warning &&
+				pstate->p_expr_kind == EXPR_KIND_SELECT_TARGET)
+			{
+				Oid			typid;
+				int32		typmod;
+				Oid			collid;
+
+				get_session_variable_type_typmod_collid(varid,
+														&typid, &typmod,
+														&collid);
+
+				/*
+				 * Some cases with ambiguous references can be solved without
+				 * raising an warning. When there is an collision between column
+				 * name (or label) and some session variable name, and when we
+				 * know attribute name, then we can ignore the collision when:
+				 *
+				 *   a) variable is of scalar type (then indirection cannot be
+				 *      applied on this session variable.
+				 *
+				 *   b) when related variable has no field of attrname, then
+				 *      indirection cannot be applied on this session variable.
+				 */
+				if (attrname)
+				{
+					if (type_is_rowtype(typid))
+					{
+						TupleDesc	tupdesc;
+						bool		found = false;
+						int			i;
+
+						/* slow part, I hope it will not be to often */
+						tupdesc = lookup_rowtype_tupdesc(typid, typmod);
+						for (i = 0; i < tupdesc->natts; i++)
+						{
+							if (namestrcmp(&(TupleDescAttr(tupdesc, i)->attname), attrname) == 0 &&
+								!TupleDescAttr(tupdesc, i)->attisdropped)
+							{
+								found = true;
+								break;
+							}
+						}
+
+						ReleaseTupleDesc(tupdesc);
+
+						/* There is no composite variable with this field. */
+						if (!found)
+							varid = InvalidOid;
+					}
+					else
+						/* There is no composite variable with this name. */
+						varid = InvalidOid;
+				}
+
+				/*
+				 * Raise warning when session variable reference is still valid.
+				 */
+				if (OidIsValid(varid))
+					ereport(WARNING,
+							(errcode(ERRCODE_AMBIGUOUS_COLUMN),
+							 errmsg("session variable \"%s\" is shadowed",
+									NameListToString(cref->fields)),
+							 errdetail("Session variables can be shadowed by columns, routine's variables and routine's arguments with same name."),
+							 parser_errposition(pstate, cref->location)));
+			}
+
+			/* Reference to session variable is shadowed by any other always. */
+			varid = InvalidOid;
+		}
+		else
+		{
+			Oid			typid;
+			int32		typmod;
+			Oid			collid;
+
+			get_session_variable_type_typmod_collid(varid, &typid, &typmod,
+													&collid);
+
+			node = makeParamSessionVariable(pstate,
+										   varid, typid, typmod, collid,
+										   attrname, cref->location);
+		}
+	}
+
 	/*
 	 * Throw error if no translation found.
 	 */
@@ -819,6 +948,74 @@ transformColumnRef(ParseState *pstate, ColumnRef *cref)
 	return node;
 }
 
+/*
+ * Generate param variable for reference to session variable
+ */
+static Node *
+makeParamSessionVariable(ParseState *pstate,
+						Oid varid, Oid typid, int32 typmod, Oid collid,
+						char *attrname, int location)
+{
+	Param	   *param;
+
+	param = makeNode(Param);
+
+	param->paramkind = PARAM_VARIABLE;
+	param->paramvarid = varid;
+	param->paramtype = typid;
+	param->paramtypmod = typmod;
+	param->paramcollid = collid;
+
+	/*
+	 * There are two ways to access session variables - direct, used by simple
+	 * plpgsql expressions, where it is not necessary to emulate stability.
+	 * And Buffered access, which is used everywhere else. We should ensure
+	 * stable values, and because session variables are global, then we should
+	 * work with copied values instead of directly accessing variables. For
+	 * direct access, the varid is best. For buffered access, we need
+	 * to assign an index to the buffer - later, when we know what variables are
+	 * used. Now, we just remember, so we use session variables.
+	 */
+	pstate->p_hasSessionVariables = true;
+
+	if (attrname != NULL)
+	{
+		TupleDesc	tupdesc;
+		int			i;
+
+		tupdesc = lookup_rowtype_tupdesc(typid, typmod);
+
+		for (i = 0; i < tupdesc->natts; i++)
+		{
+			Form_pg_attribute att = TupleDescAttr(tupdesc, i);
+
+			if (strcmp(attrname, NameStr(att->attname)) == 0 &&
+				!att->attisdropped)
+			{
+				/* Success, so generate a FieldSelect expression */
+				FieldSelect *fselect = makeNode(FieldSelect);
+
+				fselect->arg = (Expr *) param;
+				fselect->fieldnum = i + 1;
+				fselect->resulttype = att->atttypid;
+				fselect->resulttypmod = att->atttypmod;
+				/* save attribute's collation for parse_collate.c */
+				fselect->resultcollid = att->attcollation;
+
+				ReleaseTupleDesc(tupdesc);
+				return (Node *) fselect;
+			}
+		}
+
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_COLUMN),
+				 errmsg("could not identify column \"%s\" in variable", attrname),
+				 parser_errposition(pstate, location)));
+	}
+
+	return (Node *) param;
+}
+
 static Node *
 transformParamRef(ParseState *pstate, ParamRef *pref)
 {
@@ -1721,6 +1918,7 @@ transformSubLink(ParseState *pstate, SubLink *sublink)
 		case EXPR_KIND_VALUES:
 		case EXPR_KIND_VALUES_SINGLE:
 		case EXPR_KIND_CYCLE_MARK:
+		case EXPR_KIND_LET_TARGET:
 			/* okay */
 			break;
 		case EXPR_KIND_CHECK_CONSTRAINT:
@@ -1729,6 +1927,7 @@ transformSubLink(ParseState *pstate, SubLink *sublink)
 			break;
 		case EXPR_KIND_COLUMN_DEFAULT:
 		case EXPR_KIND_FUNCTION_DEFAULT:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 			err = _("cannot use subquery in DEFAULT expression");
 			break;
 		case EXPR_KIND_INDEX_EXPRESSION:
@@ -3059,6 +3258,7 @@ ParseExprKindName(ParseExprKind exprKind)
 			return "CHECK";
 		case EXPR_KIND_COLUMN_DEFAULT:
 		case EXPR_KIND_FUNCTION_DEFAULT:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 			return "DEFAULT";
 		case EXPR_KIND_INDEX_EXPRESSION:
 			return "index expression";
@@ -3084,6 +3284,8 @@ ParseExprKindName(ParseExprKind exprKind)
 			return "GENERATED AS";
 		case EXPR_KIND_CYCLE_MARK:
 			return "CYCLE";
+		case EXPR_KIND_LET_TARGET:
+			return "LET";
 
 			/*
 			 * There is intentionally no default: case here, so that the
diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index d91951e1f6..5c27abf321 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -2617,6 +2617,7 @@ check_srf_call_placement(ParseState *pstate, Node *last_srf, int location)
 			break;
 		case EXPR_KIND_COLUMN_DEFAULT:
 		case EXPR_KIND_FUNCTION_DEFAULT:
+		case EXPR_KIND_VARIABLE_DEFAULT:
 			err = _("set-returning functions are not allowed in DEFAULT expressions");
 			break;
 		case EXPR_KIND_INDEX_EXPRESSION:
@@ -2655,6 +2656,9 @@ check_srf_call_placement(ParseState *pstate, Node *last_srf, int location)
 		case EXPR_KIND_CYCLE_MARK:
 			errkind = true;
 			break;
+		case EXPR_KIND_LET_TARGET:
+			err = _("set-returning functions are not allowed in CALL arguments");
+			break;
 
 			/*
 			 * There is intentionally no default: case here, so that the
diff --git a/src/backend/parser/parse_target.c b/src/backend/parser/parse_target.c
index 059eeb9e94..bb6dab1b4b 100644
--- a/src/backend/parser/parse_target.c
+++ b/src/backend/parser/parse_target.c
@@ -89,7 +89,9 @@ transformTargetEntry(ParseState *pstate,
 		 * through unmodified.  (transformExpr will throw the appropriate
 		 * error if we're disallowing it.)
 		 */
-		if (exprKind == EXPR_KIND_UPDATE_SOURCE && IsA(node, SetToDefault))
+		if ((exprKind == EXPR_KIND_UPDATE_SOURCE ||
+			 exprKind == EXPR_KIND_LET_TARGET)
+			&& IsA(node, SetToDefault))
 			expr = node;
 		else
 			expr = transformExpr(pstate, node, exprKind);
diff --git a/src/backend/parser/parser.c b/src/backend/parser/parser.c
index 50227cc098..db2815e3be 100644
--- a/src/backend/parser/parser.c
+++ b/src/backend/parser/parser.c
@@ -61,7 +61,8 @@ raw_parser(const char *str, RawParseMode mode)
 			MODE_PLPGSQL_EXPR,	/* RAW_PARSE_PLPGSQL_EXPR */
 			MODE_PLPGSQL_ASSIGN1,	/* RAW_PARSE_PLPGSQL_ASSIGN1 */
 			MODE_PLPGSQL_ASSIGN2,	/* RAW_PARSE_PLPGSQL_ASSIGN2 */
-			MODE_PLPGSQL_ASSIGN3	/* RAW_PARSE_PLPGSQL_ASSIGN3 */
+			MODE_PLPGSQL_ASSIGN3,	/* RAW_PARSE_PLPGSQL_ASSIGN3 */
+			MODE_PLPGSQL_LET	/* RAW_PARSE_PLPGSQL_LET */
 		};
 
 		yyextra.have_lookahead = true;
diff --git a/src/backend/rewrite/rewriteHandler.c b/src/backend/rewrite/rewriteHandler.c
index 3d82138cb3..e81fd56c75 100644
--- a/src/backend/rewrite/rewriteHandler.c
+++ b/src/backend/rewrite/rewriteHandler.c
@@ -25,6 +25,7 @@
 #include "access/table.h"
 #include "catalog/dependency.h"
 #include "catalog/pg_type.h"
+#include "catalog/pg_variable.h"
 #include "commands/trigger.h"
 #include "foreign/fdwapi.h"
 #include "miscadmin.h"
@@ -3664,6 +3665,39 @@ RewriteQuery(Query *parsetree, List *rewrite_events)
 		}
 	}
 
+	/*
+	 * Rewrite SetToDefault by default expression of Let statement.
+	 */
+	if (event == CMD_SELECT && OidIsValid(parsetree->resultVariable))
+	{
+		Oid			resultVariable = parsetree->resultVariable;
+
+		if (list_length(parsetree->targetList) == 1 &&
+			IsA(((TargetEntry *) linitial(parsetree->targetList))->expr, SetToDefault))
+		{
+			Variable		var;
+			TargetEntry	   *tle;
+			TargetEntry	   *newtle;
+			Expr		   *defexpr;
+
+			/* Read session variable metadata with defexpr */
+			initVariable(&var, resultVariable, false, false);
+
+			if (var.has_defexpr)
+				defexpr = (Expr *) var.defexpr;
+			else
+				defexpr = (Expr *) makeNullConst(var.typid, var.typmod, var.collation);
+
+			tle = (TargetEntry *) linitial(parsetree->targetList);
+			newtle = makeTargetEntry(defexpr,
+									  tle->resno,
+									  pstrdup(tle->resname),
+									  false);
+
+			parsetree->targetList = list_make1(newtle);
+		}
+	}
+
 	/*
 	 * If the statement is an insert, update, or delete, adjust its targetlist
 	 * as needed, and then fire INSERT/UPDATE/DELETE rules on it.
diff --git a/src/backend/rewrite/rowsecurity.c b/src/backend/rewrite/rowsecurity.c
index f0a046d65a..06bf6c59d2 100644
--- a/src/backend/rewrite/rowsecurity.c
+++ b/src/backend/rewrite/rowsecurity.c
@@ -213,10 +213,10 @@ get_row_security_policies(Query *root, RangeTblEntry *rte, int rt_index,
 	}
 
 	/*
-	 * For SELECT, UPDATE and DELETE, add security quals to enforce the USING
-	 * policies.  These security quals control access to existing table rows.
-	 * Restrictive policies are combined together using AND, and permissive
-	 * policies are combined together using OR.
+	 * For SELECT, LET, UPDATE and DELETE, add security quals to enforce the
+	 * USING policies.  These security quals control access to existing table
+	 * rows. Restrictive policies are combined together using AND, and
+	 * permissive policies are combined together using OR.
 	 */
 
 	get_policies_for_relation(rel, commandType, user_id, &permissive_policies,
diff --git a/src/backend/tcop/dest.c b/src/backend/tcop/dest.c
index c952cbea8b..9c5339804b 100644
--- a/src/backend/tcop/dest.c
+++ b/src/backend/tcop/dest.c
@@ -37,6 +37,7 @@
 #include "executor/functions.h"
 #include "executor/tqueue.h"
 #include "executor/tstoreReceiver.h"
+#include "executor/svariableReceiver.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "utils/portal.h"
@@ -152,6 +153,9 @@ CreateDestReceiver(CommandDest dest)
 
 		case DestTupleQueue:
 			return CreateTupleQueueDestReceiver(NULL);
+
+		case DestVariable:
+			return CreateVariableDestReceiver();
 	}
 
 	/* should never get here */
@@ -207,6 +211,7 @@ EndCommand(const QueryCompletion *qc, CommandDest dest, bool force_undecorated_o
 		case DestSQLFunction:
 		case DestTransientRel:
 		case DestTupleQueue:
+		case DestVariable:
 			break;
 	}
 }
@@ -252,6 +257,7 @@ NullCommand(CommandDest dest)
 		case DestSQLFunction:
 		case DestTransientRel:
 		case DestTupleQueue:
+		case DestVariable:
 			break;
 	}
 }
@@ -295,6 +301,7 @@ ReadyForQuery(CommandDest dest)
 		case DestSQLFunction:
 		case DestTransientRel:
 		case DestTupleQueue:
+		case DestVariable:
 			break;
 	}
 }
diff --git a/src/backend/tcop/pquery.c b/src/backend/tcop/pquery.c
index 5f907831a3..4cb7403bc2 100644
--- a/src/backend/tcop/pquery.c
+++ b/src/backend/tcop/pquery.c
@@ -86,6 +86,9 @@ CreateQueryDesc(PlannedStmt *plannedstmt,
 	qd->queryEnv = queryEnv;
 	qd->instrument_options = instrument_options;	/* instrumentation wanted? */
 
+	qd->num_session_variables = 0;
+	qd->session_variables = NULL;
+
 	/* null these fields until set by ExecutorStart */
 	qd->tupDesc = NULL;
 	qd->estate = NULL;
diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
index 83e4e37c78..808a0f75b6 100644
--- a/src/backend/tcop/utility.c
+++ b/src/backend/tcop/utility.c
@@ -49,6 +49,7 @@
 #include "commands/proclang.h"
 #include "commands/publicationcmds.h"
 #include "commands/schemacmds.h"
+#include "commands/session_variable.h"
 #include "commands/seclabel.h"
 #include "commands/sequence.h"
 #include "commands/subscriptioncmds.h"
@@ -187,6 +188,7 @@ ClassifyUtilityCommandAsReadOnly(Node *parsetree)
 		case T_CreateRangeStmt:
 		case T_CreateRoleStmt:
 		case T_CreateSchemaStmt:
+		case T_CreateSessionVarStmt:
 		case T_CreateSeqStmt:
 		case T_CreateStatsStmt:
 		case T_CreateStmt:
@@ -238,6 +240,7 @@ ClassifyUtilityCommandAsReadOnly(Node *parsetree)
 
 		case T_CallStmt:
 		case T_DoStmt:
+		case T_LetStmt:
 			{
 				/*
 				 * Commands inside the DO block or the called procedure might
@@ -1062,6 +1065,11 @@ standard_ProcessUtility(PlannedStmt *pstmt,
 				break;
 			}
 
+		case T_LetStmt:
+			ExecuteLetStmt(pstate, (LetStmt *) parsetree, params,
+						   queryEnv, qc);
+			break;
+
 		default:
 			/* All other statement types have event trigger support */
 			ProcessUtilitySlow(pstate, pstmt, queryString,
@@ -1386,6 +1394,10 @@ ProcessUtilitySlow(ParseState *pstate,
 				}
 				break;
 
+			case T_CreateSessionVarStmt:
+				address = DefineSessionVariable(pstate, (CreateSessionVarStmt *) parsetree);
+				break;
+
 				/*
 				 * ************* object creation / destruction **************
 				 */
@@ -2176,6 +2188,10 @@ UtilityContainsQuery(Node *parsetree)
 				return UtilityContainsQuery(qry->utilityStmt);
 			return qry;
 
+		case T_LetStmt:
+			qry = castNode(Query, ((LetStmt *) parsetree)->query);
+			return qry;
+
 		default:
 			return NULL;
 	}
@@ -2317,6 +2333,9 @@ AlterObjectTypeCommandTag(ObjectType objtype)
 		case OBJECT_STATISTIC_EXT:
 			tag = CMDTAG_ALTER_STATISTICS;
 			break;
+		case OBJECT_VARIABLE:
+			tag = CMDTAG_ALTER_VARIABLE;
+			break;
 		default:
 			tag = CMDTAG_UNKNOWN;
 			break;
@@ -2367,6 +2386,10 @@ CreateCommandTag(Node *parsetree)
 			tag = CMDTAG_SELECT;
 			break;
 
+		case T_LetStmt:
+			tag = CMDTAG_LET;
+			break;
+
 			/* utility statements --- same whether raw or cooked */
 		case T_TransactionStmt:
 			{
@@ -2621,6 +2644,9 @@ CreateCommandTag(Node *parsetree)
 				case OBJECT_STATISTIC_EXT:
 					tag = CMDTAG_DROP_STATISTICS;
 					break;
+				case OBJECT_VARIABLE:
+					tag = CMDTAG_DROP_VARIABLE;
+					break;
 				default:
 					tag = CMDTAG_UNKNOWN;
 			}
@@ -2909,6 +2935,9 @@ CreateCommandTag(Node *parsetree)
 				case DISCARD_SEQUENCES:
 					tag = CMDTAG_DISCARD_SEQUENCES;
 					break;
+				case DISCARD_VARIABLES:
+					tag = CMDTAG_DISCARD_VARIABLES;
+					break;
 				default:
 					tag = CMDTAG_UNKNOWN;
 			}
@@ -3193,6 +3222,10 @@ CreateCommandTag(Node *parsetree)
 			}
 			break;
 
+		case T_CreateSessionVarStmt:
+			tag = CMDTAG_CREATE_VARIABLE;
+			break;
+
 		default:
 			elog(WARNING, "unrecognized node type: %d",
 				 (int) nodeTag(parsetree));
@@ -3240,6 +3273,7 @@ GetCommandLogLevel(Node *parsetree)
 			break;
 
 		case T_PLAssignStmt:
+		case T_LetStmt:
 			lev = LOGSTMT_ALL;
 			break;
 
diff --git a/src/backend/utils/adt/acl.c b/src/backend/utils/adt/acl.c
index 0a16f8156c..d767d69d22 100644
--- a/src/backend/utils/adt/acl.c
+++ b/src/backend/utils/adt/acl.c
@@ -306,6 +306,12 @@ aclparse(const char *s, AclItem *aip)
 			case ACL_CONNECT_CHR:
 				read = ACL_CONNECT;
 				break;
+			case ACL_READ_CHR:
+				read = ACL_READ;
+				break;
+			case ACL_WRITE_CHR:
+				read = ACL_WRITE;
+				break;
 			case 'R':			/* ignore old RULE privileges */
 				read = 0;
 				break;
@@ -794,6 +800,10 @@ acldefault(ObjectType objtype, Oid ownerId)
 			world_default = ACL_USAGE;
 			owner_default = ACL_ALL_RIGHTS_TYPE;
 			break;
+		case OBJECT_VARIABLE:
+			world_default = ACL_NO_RIGHTS;
+			owner_default = ACL_ALL_RIGHTS_VARIABLE;
+			break;
 		default:
 			elog(ERROR, "unrecognized objtype: %d", (int) objtype);
 			world_default = ACL_NO_RIGHTS;	/* keep compiler quiet */
@@ -888,6 +898,9 @@ acldefault_sql(PG_FUNCTION_ARGS)
 		case 'T':
 			objtype = OBJECT_TYPE;
 			break;
+		case 'V':
+			objtype = OBJECT_VARIABLE;
+			break;
 		default:
 			elog(ERROR, "unrecognized objtype abbreviation: %c", objtypec);
 	}
@@ -1604,6 +1617,10 @@ convert_priv_string(text *priv_type_text)
 		return ACL_CONNECT;
 	if (pg_strcasecmp(priv_type, "RULE") == 0)
 		return 0;				/* ignore old RULE privileges */
+	if (pg_strcasecmp(priv_type, "READ") == 0)
+		return ACL_READ;
+	if (pg_strcasecmp(priv_type, "WRITE") == 0)
+		return ACL_WRITE;
 
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
@@ -1698,6 +1715,10 @@ convert_aclright_to_string(int aclright)
 			return "TEMPORARY";
 		case ACL_CONNECT:
 			return "CONNECT";
+		case ACL_READ:
+			return "READ";
+		case ACL_WRITE:
+			return "WRITE";
 		default:
 			elog(ERROR, "unrecognized aclright: %d", aclright);
 			return NULL;
diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 039b1d2b95..c2a236e587 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -38,6 +38,7 @@
 #include "catalog/pg_statistic_ext.h"
 #include "catalog/pg_trigger.h"
 #include "catalog/pg_type.h"
+#include "catalog/pg_variable.h"
 #include "commands/defrem.h"
 #include "commands/tablespace.h"
 #include "common/keywords.h"
@@ -7969,6 +7970,14 @@ get_parameter(Param *param, deparse_context *context)
 		return;
 	}
 
+	/* translate paramvarid to session variable name */
+	if (param->paramkind == PARAM_VARIABLE)
+	{
+		appendStringInfo(context->buf, "%s",
+						 session_variable_get_name(param->paramvarid));
+		return;
+	}
+
 	/*
 	 * If it's an external parameter, see if the outermost namespace provides
 	 * function argument names.
diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index feef999863..c2755a7b04 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -35,6 +35,7 @@
 #include "catalog/pg_statistic.h"
 #include "catalog/pg_transform.h"
 #include "catalog/pg_type.h"
+#include "catalog/pg_variable.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
 #include "utils/array.h"
@@ -1860,6 +1861,18 @@ get_relname_relid(const char *relname, Oid relnamespace)
 						   ObjectIdGetDatum(relnamespace));
 }
 
+/*
+ * get_varname_varid
+ *		Given name and namespace of variable, look up the OID.
+ */
+Oid
+get_varname_varid(const char *varname, Oid varnamespace)
+{
+	return GetSysCacheOid2(VARIABLENAMENSP, Anum_pg_variable_oid,
+						   PointerGetDatum(varname),
+						   ObjectIdGetDatum(varnamespace));
+}
+
 #ifdef NOT_USED
 /*
  * get_relnatts
diff --git a/src/backend/utils/cache/plancache.c b/src/backend/utils/cache/plancache.c
index 4a9055e6bb..79ba8f3d0b 100644
--- a/src/backend/utils/cache/plancache.c
+++ b/src/backend/utils/cache/plancache.c
@@ -58,6 +58,7 @@
 
 #include "access/transam.h"
 #include "catalog/namespace.h"
+#include "catalog/pg_variable.h"
 #include "executor/executor.h"
 #include "miscadmin.h"
 #include "nodes/nodeFuncs.h"
@@ -134,6 +135,7 @@ InitPlanCache(void)
 	CacheRegisterSyscacheCallback(AMOPOPID, PlanCacheSysCallback, (Datum) 0);
 	CacheRegisterSyscacheCallback(FOREIGNSERVEROID, PlanCacheSysCallback, (Datum) 0);
 	CacheRegisterSyscacheCallback(FOREIGNDATAWRAPPEROID, PlanCacheSysCallback, (Datum) 0);
+	CacheRegisterSyscacheCallback(VARIABLEOID, PlanCacheObjectCallback, (Datum) 0);
 }
 
 /*
@@ -1867,12 +1869,24 @@ ScanQueryForLocks(Query *parsetree, bool acquire)
 	 * Recurse into sublink subqueries, too.  But we already did the ones in
 	 * the rtable and cteList.
 	 */
-	if (parsetree->hasSubLinks)
+	if (parsetree->hasSubLinks ||
+		parsetree->hasSessionVariables)
 	{
 		query_tree_walker(parsetree, ScanQueryWalker,
 						  (void *) &acquire,
 						  QTW_IGNORE_RC_SUBQUERIES);
 	}
+
+	/* Process session variables */
+	if (OidIsValid(parsetree->resultVariable))
+	{
+		if (acquire)
+			LockDatabaseObject(VariableRelationId, parsetree->resultVariable,
+							   0, AccessShareLock);
+		else
+			UnlockDatabaseObject(VariableRelationId, parsetree->resultVariable,
+							   0, AccessShareLock);
+	}
 }
 
 /*
@@ -1891,6 +1905,20 @@ ScanQueryWalker(Node *node, bool *acquire)
 		ScanQueryForLocks(castNode(Query, sub->subselect), *acquire);
 		/* Fall through to process lefthand args of SubLink */
 	}
+	else if (IsA(node, Param))
+	{
+		Param *p = (Param *) node;
+
+		if (p->paramkind == PARAM_VARIABLE)
+		{
+			if (acquire)
+				LockDatabaseObject(VariableRelationId, p->paramvarid,
+								   0, AccessShareLock);
+			else
+				UnlockDatabaseObject(VariableRelationId, p->paramvarid,
+								   0, AccessShareLock);
+		}
+	}
 
 	/*
 	 * Do NOT recurse into Query nodes, because ScanQueryForLocks already
@@ -2022,7 +2050,9 @@ PlanCacheRelCallback(Datum arg, Oid relid)
 
 /*
  * PlanCacheObjectCallback
- *		Syscache inval callback function for PROCOID and TYPEOID caches
+ *		Syscache inval callback function for TYPEOID, PROCOID, NAMESPACEOID,
+ * OPEROID, AMOPOPID, FOREIGNSERVEROID, FOREIGNDATAWRAPPEROID and VARIABLEOID
+ * caches.
  *
  * Invalidate all plans mentioning the object with the specified hash value,
  * or all plans mentioning any member of this cache if hashvalue == 0.
diff --git a/src/backend/utils/cache/syscache.c b/src/backend/utils/cache/syscache.c
index f4e7819f1e..89a8cf7d02 100644
--- a/src/backend/utils/cache/syscache.c
+++ b/src/backend/utils/cache/syscache.c
@@ -74,6 +74,7 @@
 #include "catalog/pg_ts_template.h"
 #include "catalog/pg_type.h"
 #include "catalog/pg_user_mapping.h"
+#include "catalog/pg_variable.h"
 #include "lib/qunique.h"
 #include "utils/catcache.h"
 #include "utils/rel.h"
@@ -1014,6 +1015,28 @@ static const struct cachedesc cacheinfo[] = {
 			0
 		},
 		2
+	},
+	{VariableRelationId,		/* VARIABLENAMENSP */
+		VariableNameNspIndexId,
+		2,
+		{
+			Anum_pg_variable_varname,
+			Anum_pg_variable_varnamespace,
+			0,
+			0
+		},
+		8
+	},
+	{VariableRelationId,		/* VARIABLEOID */
+		VariableObjectIndexId,
+		1,
+		{
+			Anum_pg_variable_oid,
+			0,
+			0,
+			0
+		},
+		8
 	}
 };
 
diff --git a/src/backend/utils/fmgr/fmgr.c b/src/backend/utils/fmgr/fmgr.c
index b2e72b3243..a1396f2352 100644
--- a/src/backend/utils/fmgr/fmgr.c
+++ b/src/backend/utils/fmgr/fmgr.c
@@ -1910,15 +1910,19 @@ get_call_expr_arg_stable(Node *expr, int argnum)
 	arg = (Node *) list_nth(args, argnum);
 
 	/*
-	 * Either a true Const or an external Param will have a value that doesn't
-	 * change during the execution of the query.  In future we might want to
-	 * consider other cases too, e.g. now().
+	 * Either a true Const or an external Param or variable will have a value
+	 * that doesn't change during the execution of the query.  In future we
+	 * might want to consider other cases too, e.g. now().
 	 */
 	if (IsA(arg, Const))
 		return true;
-	if (IsA(arg, Param) &&
-		((Param *) arg)->paramkind == PARAM_EXTERN)
-		return true;
+	if (IsA(arg, Param))
+	{
+		Param	   *p = (Param *) arg;
+
+		if (p->paramkind == PARAM_EXTERN || p->paramkind == PARAM_VARIABLE)
+			return true;
+	}
 
 	return false;
 }
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 4c94f09c64..3d73bf56df 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1679,6 +1679,17 @@ static struct config_bool ConfigureNamesBool[] =
 		false,
 		NULL, NULL, NULL
 	},
+
+	{
+		{"session_variables_ambiguity_warning", PGC_USERSET, DEVELOPER_OPTIONS,
+			gettext_noop("Raise warning when reference to session variable is ambiguous."),
+			NULL,
+		},
+		&session_variables_ambiguity_warning,
+		false,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"db_user_namespace", PGC_SIGHUP, CONN_AUTH_AUTH,
 			gettext_noop("Enables per-database user names."),
diff --git a/src/bin/pg_dump/common.c b/src/bin/pg_dump/common.c
index b9a25442f5..c9d959cd10 100644
--- a/src/bin/pg_dump/common.c
+++ b/src/bin/pg_dump/common.c
@@ -263,7 +263,8 @@ getSchemaData(Archive *fout, int *numTablesPtr)
 	pg_log_info("reading subscriptions");
 	getSubscriptions(fout);
 
-	free(inhinfo);				/* not needed any longer */
+	pg_log_info("reading variables");
+	getVariables(fout);
 
 	*numTablesPtr = numTables;
 	return tblinfo;
diff --git a/src/bin/pg_dump/dumputils.c b/src/bin/pg_dump/dumputils.c
index 6086d57cf3..fc90b76929 100644
--- a/src/bin/pg_dump/dumputils.c
+++ b/src/bin/pg_dump/dumputils.c
@@ -506,6 +506,11 @@ do { \
 		CONVERT_PRIV('r', "SELECT");
 		CONVERT_PRIV('w', "UPDATE");
 	}
+	else if (strcmp(type, "VARIABLE") == 0)
+	{
+		CONVERT_PRIV('S', "READ");
+		CONVERT_PRIV('W', "WRITE");
+	}
 	else
 		abort();
 
diff --git a/src/bin/pg_dump/pg_backup.h b/src/bin/pg_dump/pg_backup.h
index fcc5f6bd05..9d675d669c 100644
--- a/src/bin/pg_dump/pg_backup.h
+++ b/src/bin/pg_dump/pg_backup.h
@@ -131,12 +131,14 @@ typedef struct _restoreOptions
 	int			selFunction;
 	int			selTrigger;
 	int			selTable;
+	int			selVariable;
 	SimpleStringList indexNames;
 	SimpleStringList functionNames;
 	SimpleStringList schemaNames;
 	SimpleStringList schemaExcludeNames;
 	SimpleStringList triggerNames;
 	SimpleStringList tableNames;
+	SimpleStringList variableNames;
 
 	int			useDB;
 	ConnParams	cparams;		/* parameters to use if useDB */
diff --git a/src/bin/pg_dump/pg_backup_archiver.c b/src/bin/pg_dump/pg_backup_archiver.c
index 49bf0907cd..3a18b934b3 100644
--- a/src/bin/pg_dump/pg_backup_archiver.c
+++ b/src/bin/pg_dump/pg_backup_archiver.c
@@ -2922,6 +2922,14 @@ _tocEntryRequired(TocEntry *te, teSection curSection, ArchiveHandle *AH)
 					!simple_string_list_member(&ropt->triggerNames, te->tag))
 					return 0;
 			}
+			else if (strcmp(te->desc, "VARIABLE") == 0)
+			{
+				if (!ropt->selVariable)
+					return 0;
+				if (ropt->variableNames.head != NULL &&
+					!simple_string_list_member(&ropt->variableNames, te->tag))
+					return 0;
+			}
 			else
 				return 0;
 		}
@@ -3415,6 +3423,7 @@ _getObjectDescription(PQExpBuffer buf, TocEntry *te)
 		strcmp(type, "TEXT SEARCH DICTIONARY") == 0 ||
 		strcmp(type, "TEXT SEARCH CONFIGURATION") == 0 ||
 		strcmp(type, "STATISTICS") == 0 ||
+		strcmp(type, "VARIABLE") == 0 ||
 	/* non-schema-specified objects */
 		strcmp(type, "DATABASE") == 0 ||
 		strcmp(type, "PROCEDURAL LANGUAGE") == 0 ||
@@ -3607,7 +3616,8 @@ _printTocEntry(ArchiveHandle *AH, TocEntry *te, bool isData)
 			strcmp(te->desc, "SERVER") == 0 ||
 			strcmp(te->desc, "STATISTICS") == 0 ||
 			strcmp(te->desc, "PUBLICATION") == 0 ||
-			strcmp(te->desc, "SUBSCRIPTION") == 0)
+			strcmp(te->desc, "SUBSCRIPTION") == 0 ||
+			strcmp(te->desc, "VARIABLE") == 0)
 		{
 			PQExpBuffer temp = createPQExpBuffer();
 
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
index e3ddf19959..de2facb645 100644
--- a/src/bin/pg_dump/pg_dump.c
+++ b/src/bin/pg_dump/pg_dump.c
@@ -284,6 +284,7 @@ static void dumpPolicy(Archive *fout, const PolicyInfo *polinfo);
 static void dumpPublication(Archive *fout, const PublicationInfo *pubinfo);
 static void dumpPublicationTable(Archive *fout, const PublicationRelInfo *pubrinfo);
 static void dumpSubscription(Archive *fout, const SubscriptionInfo *subinfo);
+static void dumpVariable(Archive *fout, const VariableInfo * varinfo);
 static void dumpDatabase(Archive *AH);
 static void dumpDatabaseConfig(Archive *AH, PQExpBuffer outbuf,
 							   const char *dbname, Oid dboid);
@@ -4533,6 +4534,232 @@ get_next_possible_free_pg_type_oid(Archive *fout, PQExpBuffer upgrade_query)
 	return next_possible_free_oid;
 }
 
+/*
+ * getVariables
+ *	  get information about variables
+ */
+void
+getVariables(Archive *fout)
+{
+	PQExpBuffer query;
+	PGresult   *res;
+	VariableInfo *varinfo;
+	int			i_tableoid;
+	int			i_oid;
+	int			i_varname;
+	int			i_varnamespace;
+	int			i_vartype;
+	int			i_vartypname;
+	int			i_vardefexpr;
+	int			i_vareoxaction;
+	int			i_varisnotnull;
+	int			i_varisimmutable;
+	int			i_varowner;
+	int			i_varcollation;
+	int			i_varacl;
+	int			i_acldefault;
+	int			i,
+				ntups;
+
+	if (fout->remoteVersion < 150000)
+		return;
+
+	query = createPQExpBuffer();
+
+	resetPQExpBuffer(query);
+
+	/* Get the variables in current database. */
+	appendPQExpBuffer(query,
+					  "SELECT v.tableoid, v.oid, v.varname,\n"
+					  "v.vareoxaction,\n"
+					  "v.varnamespace,\n"
+					  "v.vartype,\n"
+					  "pg_catalog.format_type(v.vartype, v.vartypmod) as vartypname,\n"
+					  "v.varisnotnull,\n"
+					  "v.varisimmutable,\n"
+					  "CASE WHEN v.varcollation <> t.typcollation "
+					  "THEN v.varcollation ELSE 0 END AS varcollation,\n"
+					  "pg_catalog.pg_get_expr(v.vardefexpr,0) as vardefexpr,\n"
+					  "v.varowner,\n"
+					  "v.varacl,\n"
+					  "acldefault('V', v.varowner) AS acldefault\n"
+					  "FROM pg_catalog.pg_variable v\n"
+					  "JOIN pg_catalog.pg_type t "
+					  "ON (v.vartype = t.oid)");
+
+	res = ExecuteSqlQuery(fout, query->data, PGRES_TUPLES_OK);
+
+	ntups = PQntuples(res);
+
+	i_tableoid = PQfnumber(res, "tableoid");
+	i_oid = PQfnumber(res, "oid");
+	i_varname = PQfnumber(res, "varname");
+	i_varnamespace = PQfnumber(res, "varnamespace");
+	i_vartype = PQfnumber(res, "vartype");
+	i_vartypname = PQfnumber(res, "vartypname");
+	i_vareoxaction = PQfnumber(res, "vareoxaction");
+	i_vardefexpr = PQfnumber(res, "vardefexpr");
+	i_varisnotnull = PQfnumber(res, "varisnotnull");
+	i_varisimmutable = PQfnumber(res, "varisimmutable");
+	i_varcollation = PQfnumber(res, "varcollation");
+
+	i_varowner = PQfnumber(res, "varowner");
+	i_varacl = PQfnumber(res, "varacl");
+	i_acldefault = PQfnumber(res, "acldefault");
+
+	varinfo = pg_malloc(ntups * sizeof(VariableInfo));
+
+	for (i = 0; i < ntups; i++)
+	{
+		TypeInfo   *vtype;
+
+		varinfo[i].dobj.objType = DO_VARIABLE;
+		varinfo[i].dobj.catId.tableoid =
+			atooid(PQgetvalue(res, i, i_tableoid));
+		varinfo[i].dobj.catId.oid = atooid(PQgetvalue(res, i, i_oid));
+		AssignDumpId(&varinfo[i].dobj);
+		varinfo[i].dobj.name = pg_strdup(PQgetvalue(res, i, i_varname));
+		varinfo[i].dobj.namespace =
+			findNamespace(atooid(PQgetvalue(res, i, i_varnamespace)));
+
+		varinfo[i].vartype = atooid(PQgetvalue(res, i, i_vartype));
+		varinfo[i].vartypname = pg_strdup(PQgetvalue(res, i, i_vartypname));
+		varinfo[i].vareoxaction = pg_strdup(PQgetvalue(res, i, i_vareoxaction));
+		varinfo[i].varisnotnull = *(PQgetvalue(res, i, i_varisnotnull)) == 't';
+		varinfo[i].varisimmutable = *(PQgetvalue(res, i, i_varisimmutable)) == 't';
+		varinfo[i].varcollation = atooid(PQgetvalue(res, i, i_varcollation));
+
+		varinfo[i].dacl.acl = pg_strdup(PQgetvalue(res, i, i_varacl));
+		varinfo[i].dacl.acldefault = pg_strdup(PQgetvalue(res, i, i_acldefault));
+		varinfo[i].dacl.privtype = 0;
+		varinfo[i].dacl.initprivs = NULL;
+		varinfo[i].rolname = getRoleName(PQgetvalue(res, i, i_varowner));
+
+		/* Decide whether we want to dump it */
+		selectDumpableObject(&(varinfo[i].dobj), fout);
+
+		/* Do not try to dump ACL if no ACL exists. */
+		if (!PQgetisnull(res, i, i_varacl))
+			varinfo[i].dobj.components |= DUMP_COMPONENT_ACL;
+
+		if (PQgetisnull(res, i, i_vardefexpr))
+			varinfo[i].vardefexpr = NULL;
+		else
+			varinfo[i].vardefexpr = pg_strdup(PQgetvalue(res, i, i_vardefexpr));
+
+		if (strlen(varinfo[i].rolname) == 0)
+			pg_log_warning("owner of variable \"%s\" appears to be invalid",
+						   varinfo[i].dobj.name);
+
+		/* Decide whether we want to dump it */
+		selectDumpableObject(&(varinfo[i].dobj), fout);
+
+		vtype = findTypeByOid(varinfo[i].vartype);
+		addObjectDependency(&varinfo[i].dobj, vtype->dobj.dumpId);
+	}
+	PQclear(res);
+
+	destroyPQExpBuffer(query);
+}
+
+/*
+ * dumpVariable
+ *	  dump the definition of the given variables
+ */
+static void
+dumpVariable(Archive *fout, const VariableInfo * varinfo)
+{
+	DumpOptions *dopt = fout->dopt;
+
+	PQExpBuffer delq;
+	PQExpBuffer query;
+	char *qualvarname;
+	const char *vartypname;
+	const char *vardefexpr;
+	const char *vareoxaction;
+	const char *varisimmutable;
+	Oid			varcollation;
+	bool		varisnotnull;
+
+	/* Skip if not to be dumped */
+	if (!varinfo->dobj.dump || dopt->dataOnly)
+		return;
+
+	delq = createPQExpBuffer();
+	query = createPQExpBuffer();
+
+	qualvarname = pg_strdup(fmtQualifiedDumpable(varinfo));
+	vartypname = varinfo->vartypname;
+	vardefexpr = varinfo->vardefexpr;
+	vareoxaction = varinfo->vareoxaction;
+	varisnotnull = varinfo->varisnotnull;
+	varisimmutable = varinfo->varisimmutable ? "IMMUTABLE " : "";
+	varcollation = varinfo->varcollation;
+
+	appendPQExpBuffer(delq, "DROP VARIABLE %s;\n",
+					  qualvarname);
+
+	appendPQExpBuffer(query, "CREATE %sVARIABLE %s AS %s",
+					  varisimmutable, qualvarname, vartypname);
+
+	if (OidIsValid(varcollation))
+	{
+		CollInfo   *coll;
+
+		coll = findCollationByOid(varcollation);
+		if (coll)
+			appendPQExpBuffer(query, " COLLATE %s",
+							  fmtQualifiedDumpable(coll));
+	}
+
+	if (varisnotnull)
+		appendPQExpBuffer(query, " NOT NULL");
+
+	if (vardefexpr)
+		appendPQExpBuffer(query, " DEFAULT %s",
+						  vardefexpr);
+
+	if (strcmp(vareoxaction, "d") == 0)
+		appendPQExpBuffer(query, " ON COMMIT DROP");
+	else if (strcmp(vareoxaction, "r") == 0)
+		appendPQExpBuffer(query, " ON TRANSACTION END RESET");
+
+	appendPQExpBuffer(query, ";\n");
+
+	if (varinfo->dobj.dump & DUMP_COMPONENT_DEFINITION)
+		ArchiveEntry(fout, varinfo->dobj.catId, varinfo->dobj.dumpId,
+					 ARCHIVE_OPTS(.tag = varinfo->dobj.name,
+								  .namespace = varinfo->dobj.namespace->dobj.name,
+								  .owner = varinfo->rolname,
+								  .description = "VARIABLE",
+								  .section = SECTION_PRE_DATA,
+								  .createStmt = query->data,
+								  .dropStmt = delq->data));
+
+	/* Dump comment if any */
+	if (varinfo->dobj.dump & DUMP_COMPONENT_COMMENT)
+		dumpComment(fout, "VARIABLE", qualvarname,
+					NULL, varinfo->rolname,
+					varinfo->dobj.catId, 0, varinfo->dobj.dumpId);
+
+	/* Dump ACL if any */
+	if (varinfo->dobj.dump & DUMP_COMPONENT_ACL)
+	{
+		char *qvarname = pg_strdup(fmtId(varinfo->dobj.name));
+
+		dumpACL(fout, varinfo->dobj.dumpId, InvalidDumpId, "VARIABLE",
+				qvarname, NULL,
+				varinfo->dobj.namespace->dobj.name, varinfo->rolname, &varinfo->dacl);
+
+		free(qvarname);
+	}
+
+	destroyPQExpBuffer(delq);
+	destroyPQExpBuffer(query);
+
+	free(qualvarname);
+}
+
 static void
 binary_upgrade_set_type_oids_by_type_oid(Archive *fout,
 										 PQExpBuffer upgrade_buffer,
@@ -9179,7 +9406,8 @@ getAdditionalACLs(Archive *fout)
 					dobj->objType == DO_TABLE ||
 					dobj->objType == DO_PROCLANG ||
 					dobj->objType == DO_FDW ||
-					dobj->objType == DO_FOREIGN_SERVER)
+					dobj->objType == DO_FOREIGN_SERVER ||
+					dobj->objType == DO_VARIABLE)
 				{
 					DumpableObjectWithAcl *daobj = (DumpableObjectWithAcl *) dobj;
 
@@ -9769,6 +9997,9 @@ dumpDumpableObject(Archive *fout, DumpableObject *dobj)
 		case DO_SUBSCRIPTION:
 			dumpSubscription(fout, (const SubscriptionInfo *) dobj);
 			break;
+		case DO_VARIABLE:
+			dumpVariable(fout, (VariableInfo *) dobj);
+			break;
 		case DO_PRE_DATA_BOUNDARY:
 		case DO_POST_DATA_BOUNDARY:
 			/* never dumped, nothing to do */
@@ -14115,6 +14346,9 @@ dumpDefaultACL(Archive *fout, const DefaultACLInfo *daclinfo)
 		case DEFACLOBJ_NAMESPACE:
 			type = "SCHEMAS";
 			break;
+		case DEFACLOBJ_VARIABLE:
+			type = "VARIABLE";
+			break;
 		default:
 			/* shouldn't get here */
 			fatal("unrecognized object type in default privileges: %d",
@@ -17660,6 +17894,7 @@ addBoundaryDependencies(DumpableObject **dobjs, int numObjs,
 			case DO_CONVERSION:
 			case DO_TABLE:
 			case DO_TABLE_ATTACH:
+			case DO_VARIABLE:
 			case DO_ATTRDEF:
 			case DO_PROCLANG:
 			case DO_CAST:
diff --git a/src/bin/pg_dump/pg_dump.h b/src/bin/pg_dump/pg_dump.h
index 066a129ee5..3ece7e89c3 100644
--- a/src/bin/pg_dump/pg_dump.h
+++ b/src/bin/pg_dump/pg_dump.h
@@ -52,6 +52,7 @@ typedef enum
 	DO_TABLE,
 	DO_TABLE_ATTACH,
 	DO_ATTRDEF,
+	DO_VARIABLE,
 	DO_INDEX,
 	DO_INDEX_ATTACH,
 	DO_STATSEXT,
@@ -82,7 +83,7 @@ typedef enum
 	DO_PUBLICATION,
 	DO_PUBLICATION_REL,
 	DO_PUBLICATION_TABLE_IN_SCHEMA,
-	DO_SUBSCRIPTION
+	DO_SUBSCRIPTION,
 } DumpableObjectType;
 
 /*
@@ -659,6 +660,27 @@ typedef struct _SubscriptionInfo
 	char	   *subpublications;
 } SubscriptionInfo;
 
+/*
+ * The VariableInfo struct is used to represent session variables
+ */
+typedef struct _VariableInfo
+{
+	DumpableObject dobj;
+	DumpableAcl dacl;
+	Oid			vartype;
+	char	   *vartypname;
+	char	   *vareoxaction;
+	char	   *vardefexpr;
+	char	   *varacl;
+	char	   *rvaracl;
+	char	   *initvaracl;
+	char	   *initrvaracl;
+	bool		varisnotnull;
+	bool		varisimmutable;
+	Oid			varcollation;
+	const char *rolname;		/* name of owner, or empty string */
+}			VariableInfo;
+
 /*
  *	common utility functions
  */
@@ -741,5 +763,6 @@ extern void getPublicationNamespaces(Archive *fout);
 extern void getPublicationTables(Archive *fout, TableInfo tblinfo[],
 								 int numTables);
 extern void getSubscriptions(Archive *fout);
+extern void getVariables(Archive *fout);
 
 #endif							/* PG_DUMP_H */
diff --git a/src/bin/pg_dump/pg_dump_sort.c b/src/bin/pg_dump/pg_dump_sort.c
index d979f93b3d..b883e92924 100644
--- a/src/bin/pg_dump/pg_dump_sort.c
+++ b/src/bin/pg_dump/pg_dump_sort.c
@@ -75,6 +75,7 @@ enum dbObjectTypePriorities
 	PRIO_TABLE_ATTACH,
 	PRIO_DUMMY_TYPE,
 	PRIO_ATTRDEF,
+	PRIO_VARIABLE,
 	PRIO_BLOB,
 	PRIO_PRE_DATA_BOUNDARY,		/* boundary! */
 	PRIO_TABLE_DATA,
@@ -116,6 +117,7 @@ static const int dbObjectTypePriority[] =
 	PRIO_TABLE,					/* DO_TABLE */
 	PRIO_TABLE_ATTACH,			/* DO_TABLE_ATTACH */
 	PRIO_ATTRDEF,				/* DO_ATTRDEF */
+	PRIO_VARIABLE,				/* DO_VARIABLE */
 	PRIO_INDEX,					/* DO_INDEX */
 	PRIO_INDEX_ATTACH,			/* DO_INDEX_ATTACH */
 	PRIO_STATSEXT,				/* DO_STATSEXT */
@@ -1508,6 +1510,10 @@ describeDumpableObject(DumpableObject *obj, char *buf, int bufsize)
 					 "POST-DATA BOUNDARY  (ID %d)",
 					 obj->dumpId);
 			return;
+		case DO_VARIABLE:
+			snprintf(buf, bufsize,
+					 "VARIABLE %s  (ID %d OID %u)",
+					 obj->name, obj->dumpId, obj->catId.oid);
 	}
 	/* shouldn't get here */
 	snprintf(buf, bufsize,
diff --git a/src/bin/pg_dump/pg_restore.c b/src/bin/pg_dump/pg_restore.c
index 55bf1b6975..d934094f8a 100644
--- a/src/bin/pg_dump/pg_restore.c
+++ b/src/bin/pg_dump/pg_restore.c
@@ -103,6 +103,7 @@ main(int argc, char **argv)
 		{"trigger", 1, NULL, 'T'},
 		{"use-list", 1, NULL, 'L'},
 		{"username", 1, NULL, 'U'},
+		{"variable", 1, NULL, 'A'},
 		{"verbose", 0, NULL, 'v'},
 		{"single-transaction", 0, NULL, '1'},
 
@@ -151,7 +152,7 @@ main(int argc, char **argv)
 		}
 	}
 
-	while ((c = getopt_long(argc, argv, "acCd:ef:F:h:I:j:lL:n:N:Op:P:RsS:t:T:U:vwWx1",
+	while ((c = getopt_long(argc, argv, "A:acCd:ef:F:h:I:j:lL:n:N:Op:P:RsS:t:T:U:vwWx1",
 							cmdopts, NULL)) != -1)
 	{
 		switch (c)
@@ -159,6 +160,11 @@ main(int argc, char **argv)
 			case 'a':			/* Dump data only */
 				opts->dataOnly = 1;
 				break;
+			case 'A':			/* vAriable */
+				opts->selTypes = 1;
+				opts->selVariable = 1;
+				simple_string_list_append(&opts->variableNames, optarg);
+				break;
 			case 'c':			/* clean (i.e., drop) schema prior to create */
 				opts->dropSchema = 1;
 				break;
@@ -464,6 +470,7 @@ usage(const char *progname)
 
 	printf(_("\nOptions controlling the restore:\n"));
 	printf(_("  -a, --data-only              restore only the data, no schema\n"));
+	printf(_("  -A, --variable=NAME          restore named session variable\n"));
 	printf(_("  -c, --clean                  clean (drop) database objects before recreating\n"));
 	printf(_("  -C, --create                 create the target database\n"));
 	printf(_("  -e, --exit-on-error          exit on error, default is to continue\n"));
diff --git a/src/bin/pg_dump/t/002_pg_dump.pl b/src/bin/pg_dump/t/002_pg_dump.pl
index 39fa1952e7..49dcbd4e15 100644
--- a/src/bin/pg_dump/t/002_pg_dump.pl
+++ b/src/bin/pg_dump/t/002_pg_dump.pl
@@ -509,6 +509,16 @@ my %tests = (
 		unlike => { no_privs => 1, },
 	  },
 
+	'ALTER DEFAULT PRIVILEGES FOR ROLE regress_dump_test_role GRANT READ ON VARIABLES TO PUBLIC'
+	  => {
+		create_order => 56,
+		create_sql   => 'ALTER DEFAULT PRIVILEGES FOR ROLE regress_dump_test_role GRANT READ ON VARIABLES TO PUBLIC;',
+		regexp => qr/^
+			\QALTER DEFAULT PRIVILEGES FOR ROLE regress_dump_test_role GRANT READ ON VARIABLE  TO PUBLIC;\E/xm,
+		like => { %full_runs, section_post_data => 1, },
+		unlike => { no_privs => 1, },
+	  },
+
 	'ALTER ROLE regress_dump_test_role' => {
 		regexp => qr/^
 			\QALTER ROLE regress_dump_test_role WITH \E
@@ -1264,6 +1274,22 @@ my %tests = (
 		unlike => { exclude_dump_test_schema => 1, },
 	},
 
+	'COMMENT ON VARIABLE dump_test.variable1' => {
+		create_order => 71,
+		create_sql   => 'COMMENT ON VARIABLE dump_test.variable1
+					   IS \'comment on variable\';',
+		regexp =>
+		  qr/^\QCOMMENT ON VARIABLE dump_test.variable1 IS 'comment on variable';\E/m,
+		like => {
+			%full_runs,
+			%dump_test_schema_runs,
+			section_pre_data     => 1,
+		},
+		unlike => {
+			exclude_dump_test_schema => 1,
+		},
+	},
+
 	'COPY test_table' => {
 		create_order => 4,
 		create_sql   => 'INSERT INTO dump_test.test_table (col1) '
@@ -3071,6 +3097,30 @@ my %tests = (
 		},
 	},
 
+	'CREATE VARIABLE test_variable' => {
+		all_runs     => 1,
+		catch_all    => 'CREATE ... commands',
+		create_order => 61,
+		create_sql   => 'CREATE VARIABLE dump_test.variable1 AS integer DEFAULT 0;',
+		regexp => qr/^
+			\QCREATE VARIABLE dump_test.variable1 AS integer DEFAULT 0;\E/xm,
+		like =>
+		  { %full_runs, %dump_test_schema_runs, section_pre_data => 1, },
+		unlike => { exclude_dump_test_schema => 1, },
+	},
+
+	'CREATE IMMUTABLE VARIABLE test_variable' => {
+		all_runs     => 1,
+		catch_all    => 'CREATE ... commands',
+		create_order => 61,
+		create_sql   => 'CREATE IMMUTABLE VARIABLE dump_test.variable2 AS integer DEFAULT 0;',
+		regexp => qr/^
+			\QCREATE IMMUTABLE VARIABLE dump_test.variable2 AS integer DEFAULT 0;\E/xm,
+		like =>
+		  { %full_runs, %dump_test_schema_runs, section_pre_data => 1, },
+		unlike => { exclude_dump_test_schema => 1, },
+	},
+
 	'CREATE VIEW test_view' => {
 		create_order => 61,
 		create_sql   => 'CREATE VIEW dump_test.test_view
@@ -3510,6 +3560,21 @@ my %tests = (
 		like => {},
 	},
 
+	'GRANT READ ON VARIABLE dump_test.variable1' => {
+		create_order => 73,
+		create_sql =>
+		  'GRANT READ ON VARIABLE dump_test.variable1 TO regress_dump_test_role;',
+		regexp => qr/^
+			\QGRANT READ ON VARIABLE dump_test.variable1 TO regress_dump_test_role;\E
+			/xm,
+		like =>
+		  { %full_runs, %dump_test_schema_runs, section_pre_data => 1, },
+		unlike => {
+			exclude_dump_test_schema => 1,
+			no_privs                 => 1,
+		},
+	},
+
 	'REFRESH MATERIALIZED VIEW matview' => {
 		regexp => qr/^\QREFRESH MATERIALIZED VIEW dump_test.matview;\E/m,
 		like =>
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index f590474855..05bde3e50e 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -940,6 +940,9 @@ exec_command_d(PsqlScanState scan_state, bool active_branch, const char *cmd)
 						break;
 				}
 				break;
+			case 'V':			/* Variables */
+				success = listVariables(pattern, show_verbose);
+				break;
 			case 'x':			/* Extensions */
 				if (show_verbose)
 					success = listExtensionContents(pattern);
diff --git a/src/bin/psql/describe.c b/src/bin/psql/describe.c
index 346cd92793..8d3fb6578b 100644
--- a/src/bin/psql/describe.c
+++ b/src/bin/psql/describe.c
@@ -4803,6 +4803,100 @@ listSchemas(const char *pattern, bool verbose, bool showSystem)
 	return true;
 }
 
+/*
+ * \dV
+ *
+ * listVariables()
+ */
+bool
+listVariables(const char *pattern, bool verbose)
+{
+	PQExpBufferData buf;
+	PGresult   *res;
+	printQueryOpt myopt = pset.popt;
+	static const bool translate_columns[] = {false, false, false, false, false, false, false, false, false, false, false};
+
+	initPQExpBuffer(&buf);
+
+	printfPQExpBuffer(&buf,
+					  "SELECT n.nspname as \"%s\",\n"
+					  "  v.varname as \"%s\",\n"
+					  "  pg_catalog.format_type(v.vartype, v.vartypmod) as \"%s\",\n"
+					  "  (SELECT c.collname FROM pg_catalog.pg_collation c, pg_catalog.pg_type bt\n"
+					  "   WHERE c.oid = v.varcollation AND bt.oid = v.vartype AND v.varcollation <> bt.typcollation) as \"%s\",\n"
+					  "  NOT v.varisnotnull as \"%s\",\n"
+					  "  NOT v.varisimmutable as \"%s\",\n"
+					  "  pg_catalog.pg_get_expr(v.vardefexpr, 0) as \"%s\",\n"
+					  "  pg_catalog.pg_get_userbyid(v.varowner) as \"%s\",\n"
+					  "  CASE v.vareoxaction\n"
+					  "    WHEN 'd' THEN 'ON COMMIT DROP'\n"
+					  "    WHEN 'r' THEN 'ON TRANSACTION END RESET' END as \"%s\"\n",
+					  gettext_noop("Schema"),
+					  gettext_noop("Name"),
+					  gettext_noop("Type"),
+					  gettext_noop("Collation"),
+					  gettext_noop("Nullable"),
+					  gettext_noop("Mutable"),
+					  gettext_noop("Default"),
+					  gettext_noop("Owner"),
+					  gettext_noop("Transactional end action"));
+
+	if (verbose)
+	{
+		appendPQExpBufferStr(&buf, ",\n  ");
+		printACLColumn(&buf, "v.varacl");
+		appendPQExpBuffer(&buf,
+						  ",\n  pg_catalog.obj_description(v.oid, 'pg_variable') AS \"%s\"",
+						  gettext_noop("Description"));
+	}
+
+	appendPQExpBufferStr(&buf,
+						 "\nFROM pg_catalog.pg_variable v"
+						 "\n     LEFT JOIN pg_catalog.pg_namespace n ON n.oid = v.varnamespace");
+
+	appendPQExpBufferStr(&buf, "\nWHERE true\n");
+	if (!pattern)
+		appendPQExpBufferStr(&buf, "      AND n.nspname <> 'pg_catalog'\n"
+							 "      AND n.nspname <> 'information_schema'\n");
+
+	processSQLNamePattern(pset.db, &buf, pattern, true, false,
+						  "n.nspname", "v.varname", NULL,
+						  "pg_catalog.pg_variable_is_visible(v.oid)");
+
+	appendPQExpBufferStr(&buf, "ORDER BY 1,2;");
+
+	res = PSQLexec(buf.data);
+	termPQExpBuffer(&buf);
+	if (!res)
+		return false;
+
+	/*
+	 * Most functions in this file are content to print an empty table when
+	 * there are no matching objects.  We intentionally deviate from that
+	 * here, but only in !quiet mode, for historical reasons.
+	 */
+	if (PQntuples(res) == 0 && !pset.quiet)
+	{
+		if (pattern)
+			pg_log_error("Did not find any session variable named \"%s\".",
+						 pattern);
+		else
+			pg_log_error("Did not find any session variables.");
+	}
+	else
+	{
+		myopt.nullPrint = NULL;
+		myopt.title = _("List of variables");
+		myopt.translate_header = true;
+		myopt.translate_columns = translate_columns;
+		myopt.n_translate_columns = lengthof(translate_columns);
+
+		printQuery(res, &myopt, pset.queryFout, false, pset.logfile);
+	}
+
+	PQclear(res);
+	return true;
+}
 
 /*
  * \dFp
diff --git a/src/bin/psql/describe.h b/src/bin/psql/describe.h
index fd6079679c..1c77f0547a 100644
--- a/src/bin/psql/describe.h
+++ b/src/bin/psql/describe.h
@@ -142,4 +142,7 @@ extern bool listOpFamilyFunctions(const char *access_method_pattern,
 /* \dl or \lo_list */
 extern bool listLargeObjects(bool verbose);
 
+/* \dV */
+extern bool listVariables(const char *pattern, bool varbose);
+
 #endif							/* DESCRIBE_H */
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index 937d6e9d49..92b55a4768 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -166,7 +166,7 @@ slashUsage(unsigned short int pager)
 	 * Use "psql --help=commands | wc" to count correctly.  It's okay to count
 	 * the USE_READLINE line even in builds without that.
 	 */
-	output = PageOutput(137, pager ? &(pset.popt.topt) : NULL);
+	output = PageOutput(138, pager ? &(pset.popt.topt) : NULL);
 
 	fprintf(output, _("General\n"));
 	fprintf(output, _("  \\copyright             show PostgreSQL usage and distribution terms\n"));
@@ -265,6 +265,7 @@ slashUsage(unsigned short int pager)
 	fprintf(output, _("  \\dT[S+] [PATTERN]      list data types\n"));
 	fprintf(output, _("  \\du[S+] [PATTERN]      list roles\n"));
 	fprintf(output, _("  \\dv[S+] [PATTERN]      list views\n"));
+	fprintf(output, _("  \\dV     [PATTERN]      list variables\n"));
 	fprintf(output, _("  \\dx[+]  [PATTERN]      list extensions\n"));
 	fprintf(output, _("  \\dX     [PATTERN]      list extended statistics\n"));
 	fprintf(output, _("  \\dy[+]  [PATTERN]      list event triggers\n"));
diff --git a/src/bin/psql/tab-complete.c b/src/bin/psql/tab-complete.c
index 502b5c5751..6f25748262 100644
--- a/src/bin/psql/tab-complete.c
+++ b/src/bin/psql/tab-complete.c
@@ -643,6 +643,13 @@ static const SchemaQuery Query_for_list_of_collations = {
 	.result = "pg_catalog.quote_ident(c.collname)",
 };
 
+static const SchemaQuery Query_for_list_of_variables = {
+	.min_server_version = 150000,
+	.catname = "pg_catalog.pg_variable v",
+	.viscondition = "pg_catalog.pg_variable_is_visible(v.oid)",
+	.namespace = "v.varnamespace",
+	.result = "pg_catalog.quote_ident(v.varname)",
+};
 
 /*
  * Queries to get lists of names of various kinds of things, possibly
@@ -1141,6 +1148,7 @@ static const pgsql_thing_t words_after_create[] = {
 																	 * TABLE ... */
 	{"USER", Query_for_list_of_roles " UNION SELECT 'MAPPING FOR'"},
 	{"USER MAPPING FOR", NULL, NULL, NULL},
+	{"VARIABLE", NULL, NULL, &Query_for_list_of_variables},
 	{"VIEW", NULL, NULL, &Query_for_list_of_views},
 	{NULL}						/* end of list */
 };
@@ -1539,8 +1547,8 @@ psql_completion(const char *text, int start, int end)
 		"ABORT", "ALTER", "ANALYZE", "BEGIN", "CALL", "CHECKPOINT", "CLOSE", "CLUSTER",
 		"COMMENT", "COMMIT", "COPY", "CREATE", "DEALLOCATE", "DECLARE",
 		"DELETE FROM", "DISCARD", "DO", "DROP", "END", "EXECUTE", "EXPLAIN",
-		"FETCH", "GRANT", "IMPORT FOREIGN SCHEMA", "INSERT INTO", "LISTEN", "LOAD", "LOCK",
-		"MOVE", "NOTIFY", "PREPARE",
+		"FETCH", "GRANT", "IMPORT FOREIGN SCHEMA", "INSERT INTO", "LET", "LISTEN", "LOAD",
+		"LOCK", "MOVE", "NOTIFY", "PREPARE",
 		"REASSIGN", "REFRESH MATERIALIZED VIEW", "REINDEX", "RELEASE",
 		"RESET", "REVOKE", "ROLLBACK",
 		"SAVEPOINT", "SECURITY LABEL", "SELECT", "SET", "SHOW", "START",
@@ -1559,7 +1567,7 @@ psql_completion(const char *text, int start, int end)
 		"\\dF", "\\dFd", "\\dFp", "\\dFt", "\\dg", "\\di", "\\dl", "\\dL",
 		"\\dm", "\\dn", "\\do", "\\dO", "\\dp", "\\dP", "\\dPi", "\\dPt",
 		"\\drds", "\\dRs", "\\dRp", "\\ds",
-		"\\dt", "\\dT", "\\dv", "\\du", "\\dx", "\\dX", "\\dy",
+		"\\dt", "\\dT", "\\dv", "\\du", "\\dx", "\\dX", "\\dy", "\\dV",
 		"\\echo", "\\edit", "\\ef", "\\elif", "\\else", "\\encoding",
 		"\\endif", "\\errverbose", "\\ev",
 		"\\f",
@@ -1976,6 +1984,9 @@ psql_completion(const char *text, int start, int end)
 		COMPLETE_WITH_QUERY(Query_for_list_of_alter_system_set_vars);
 	else if (Matches("ALTER", "SYSTEM", "SET", MatchAny))
 		COMPLETE_WITH("TO");
+	/* ALTER VARIABLE <name> */
+	else if (Matches("ALTER", "VARIABLE", MatchAny))
+		COMPLETE_WITH("OWNER TO", "RENAME TO", "SET SCHEMA");
 	/* ALTER VIEW <name> */
 	else if (Matches("ALTER", "VIEW", MatchAny))
 		COMPLETE_WITH("ALTER COLUMN", "OWNER TO", "RENAME",
@@ -2477,7 +2488,7 @@ psql_completion(const char *text, int start, int end)
 					  "ROUTINE", "RULE", "SCHEMA", "SEQUENCE", "SERVER",
 					  "STATISTICS", "SUBSCRIPTION", "TABLE",
 					  "TABLESPACE", "TEXT SEARCH", "TRANSFORM FOR",
-					  "TRIGGER", "TYPE", "VIEW");
+					  "TRIGGER", "TYPE", "VARIABLE", "VIEW");
 	else if (Matches("COMMENT", "ON", "ACCESS", "METHOD"))
 		COMPLETE_WITH_QUERY(Query_for_list_of_access_methods);
 	else if (Matches("COMMENT", "ON", "CONSTRAINT"))
@@ -2903,7 +2914,7 @@ psql_completion(const char *text, int start, int end)
 /* CREATE TABLE --- is allowed inside CREATE SCHEMA, so use TailMatches */
 	/* Complete "CREATE TEMP/TEMPORARY" with the possible temp objects */
 	else if (TailMatches("CREATE", "TEMP|TEMPORARY"))
-		COMPLETE_WITH("SEQUENCE", "TABLE", "VIEW");
+		COMPLETE_WITH("SEQUENCE", "TABLE", "VIEW", "VARIABLE");
 	/* Complete "CREATE UNLOGGED" with TABLE or MATVIEW */
 	else if (TailMatches("CREATE", "UNLOGGED"))
 		COMPLETE_WITH("TABLE", "MATERIALIZED VIEW");
@@ -3210,6 +3221,14 @@ psql_completion(const char *text, int start, int end)
 		else if (TailMatches("=", MatchAnyExcept("*)")))
 			COMPLETE_WITH(",", ")");
 	}
+/* CREATE VARIABLE --- is allowed inside CREATE SCHEMA, so use TailMatches */
+	/* Complete CREATE VARIABLE <name> with AS */
+	else if (TailMatches("CREATE", "VARIABLE", MatchAny) ||
+			 TailMatches("TEMP|TEMPORARY", "VARIABLE", MatchAny))
+			COMPLETE_WITH("AS");
+	else if (TailMatches("VARIABLE", MatchAny, "AS"))
+			/* Complete CREATE VARIABLE <name> with AS types */
+			COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_datatypes, NULL);
 
 /* CREATE VIEW --- is allowed inside CREATE SCHEMA, so use TailMatches */
 	/* Complete CREATE [ OR REPLACE ] VIEW <name> with AS */
@@ -3324,7 +3343,7 @@ psql_completion(const char *text, int start, int end)
 
 /* DISCARD */
 	else if (Matches("DISCARD"))
-		COMPLETE_WITH("ALL", "PLANS", "SEQUENCES", "TEMP");
+		COMPLETE_WITH("ALL", "PLANS", "SEQUENCES", "TEMP", "VARIABLES");
 
 /* DO */
 	else if (Matches("DO"))
@@ -3451,6 +3470,12 @@ psql_completion(const char *text, int start, int end)
 	else if (Matches("DROP", "TRANSFORM", "FOR", MatchAny, "LANGUAGE", MatchAny))
 		COMPLETE_WITH("CASCADE", "RESTRICT");
 
+	/* DROP VARIABLE */
+	else if (Matches("DROP", "VARIABLE"))
+		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_variables, NULL);
+	else if (Matches("DROP", "VARIABLE", MatchAny))
+		COMPLETE_WITH("CASCADE", "RESTRICT");
+
 /* EXECUTE */
 	else if (Matches("EXECUTE"))
 		COMPLETE_WITH_QUERY(Query_for_list_of_prepared_statements);
@@ -3566,7 +3591,8 @@ psql_completion(const char *text, int start, int end)
 		if (HeadMatches("ALTER", "DEFAULT", "PRIVILEGES"))
 			COMPLETE_WITH("SELECT", "INSERT", "UPDATE",
 						  "DELETE", "TRUNCATE", "REFERENCES", "TRIGGER",
-						  "EXECUTE", "USAGE", "ALL");
+						  "EXECUTE", "USAGE", "ALL",
+						  "READ", "WRITE");
 		else
 			COMPLETE_WITH_QUERY(Query_for_list_of_roles
 								" UNION SELECT 'SELECT'"
@@ -3581,6 +3607,8 @@ psql_completion(const char *text, int start, int end)
 								" UNION SELECT 'TEMPORARY'"
 								" UNION SELECT 'EXECUTE'"
 								" UNION SELECT 'USAGE'"
+								" UNION SELECT 'READ'"
+								" UNION SELECT 'WRITE'"
 								" UNION SELECT 'ALL'");
 	}
 
@@ -3590,7 +3618,7 @@ psql_completion(const char *text, int start, int end)
 	 */
 	else if (TailMatches("GRANT|REVOKE", MatchAny))
 	{
-		if (TailMatches("SELECT|INSERT|UPDATE|DELETE|TRUNCATE|REFERENCES|TRIGGER|CREATE|CONNECT|TEMPORARY|TEMP|EXECUTE|USAGE|ALL"))
+		if (TailMatches("SELECT|INSERT|UPDATE|DELETE|TRUNCATE|REFERENCES|TRIGGER|CREATE|CONNECT|TEMPORARY|TEMP|EXECUTE|USAGE|READ|WRITE|ALL"))
 			COMPLETE_WITH("ON");
 		else if (TailMatches("GRANT", MatchAny))
 			COMPLETE_WITH("TO");
@@ -3615,7 +3643,7 @@ psql_completion(const char *text, int start, int end)
 		 * objects supported.
 		 */
 		if (HeadMatches("ALTER", "DEFAULT", "PRIVILEGES"))
-			COMPLETE_WITH("TABLES", "SEQUENCES", "FUNCTIONS", "PROCEDURES", "ROUTINES", "TYPES", "SCHEMAS");
+			COMPLETE_WITH("TABLES", "SEQUENCES", "FUNCTIONS", "PROCEDURES", "ROUTINES", "TYPES", "SCHEMAS", "VARIABLES");
 		else
 			COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_grantables,
 									   " UNION SELECT 'ALL FUNCTIONS IN SCHEMA'"
@@ -3623,6 +3651,7 @@ psql_completion(const char *text, int start, int end)
 									   " UNION SELECT 'ALL ROUTINES IN SCHEMA'"
 									   " UNION SELECT 'ALL SEQUENCES IN SCHEMA'"
 									   " UNION SELECT 'ALL TABLES IN SCHEMA'"
+									   " UNION SELECT 'ALL VARIABLES IN SCHEMA'"
 									   " UNION SELECT 'DATABASE'"
 									   " UNION SELECT 'DOMAIN'"
 									   " UNION SELECT 'FOREIGN DATA WRAPPER'"
@@ -3636,14 +3665,16 @@ psql_completion(const char *text, int start, int end)
 									   " UNION SELECT 'SEQUENCE'"
 									   " UNION SELECT 'TABLE'"
 									   " UNION SELECT 'TABLESPACE'"
-									   " UNION SELECT 'TYPE'");
+									   " UNION SELECT 'TYPE'"
+									   " UNION SELECT 'VARIABLE'");
 	}
 	else if (TailMatches("GRANT|REVOKE", MatchAny, "ON", "ALL"))
 		COMPLETE_WITH("FUNCTIONS IN SCHEMA",
 					  "PROCEDURES IN SCHEMA",
 					  "ROUTINES IN SCHEMA",
 					  "SEQUENCES IN SCHEMA",
-					  "TABLES IN SCHEMA");
+					  "TABLES IN SCHEMA",
+					  "VARIABLES IN SCHEMA");
 	else if (TailMatches("GRANT|REVOKE", MatchAny, "ON", "FOREIGN"))
 		COMPLETE_WITH("DATA WRAPPER", "SERVER");
 
@@ -3677,6 +3708,8 @@ psql_completion(const char *text, int start, int end)
 			COMPLETE_WITH_QUERY(Query_for_list_of_tablespaces);
 		else if (TailMatches("TYPE"))
 			COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_datatypes, NULL);
+		else if (TailMatches("VARIABLE"))
+			COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_variables, NULL);
 		else if (TailMatches("GRANT", MatchAny, MatchAny, MatchAny))
 			COMPLETE_WITH("TO");
 		else
@@ -3849,7 +3882,7 @@ psql_completion(const char *text, int start, int end)
 
 /* PREPARE xx AS */
 	else if (Matches("PREPARE", MatchAny, "AS"))
-		COMPLETE_WITH("SELECT", "UPDATE", "INSERT INTO", "DELETE FROM");
+		COMPLETE_WITH("SELECT", "UPDATE", "INSERT INTO", "DELETE FROM", "LET");
 
 /*
  * PREPARE TRANSACTION is missing on purpose. It's intended for transaction
@@ -4123,6 +4156,14 @@ psql_completion(const char *text, int start, int end)
 	else if (TailMatches("UPDATE", MatchAny, "SET", MatchAnyExcept("*=")))
 		COMPLETE_WITH("=");
 
+/* LET --- can be inside EXPLAIN, PREPARE etc */
+	/* If prev. word is LET suggest a list of variables */
+	else if (TailMatches("LET"))
+		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_variables, NULL);
+	/* Complete LET <variable> with "=" */
+	else if (TailMatches("LET", MatchAny))
+		COMPLETE_WITH("=");
+
 /* USER MAPPING */
 	else if (Matches("ALTER|CREATE|DROP", "USER", "MAPPING"))
 		COMPLETE_WITH("FOR");
@@ -4287,6 +4328,8 @@ psql_completion(const char *text, int start, int end)
 		COMPLETE_WITH_QUERY(Query_for_list_of_roles);
 	else if (TailMatchesCS("\\dv*"))
 		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_views, NULL);
+	else if (TailMatchesCS("\\dV*"))
+		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_variables, NULL);
 	else if (TailMatchesCS("\\dx*"))
 		COMPLETE_WITH_QUERY(Query_for_list_of_extensions);
 	else if (TailMatchesCS("\\dX*"))
diff --git a/src/include/catalog/dependency.h b/src/include/catalog/dependency.h
index 344482ec87..b5aec6b519 100644
--- a/src/include/catalog/dependency.h
+++ b/src/include/catalog/dependency.h
@@ -125,10 +125,11 @@ typedef enum ObjectClass
 	OCLASS_PUBLICATION_NAMESPACE,	/* pg_publication_namespace */
 	OCLASS_PUBLICATION_REL,		/* pg_publication_rel */
 	OCLASS_SUBSCRIPTION,		/* pg_subscription */
-	OCLASS_TRANSFORM			/* pg_transform */
+	OCLASS_TRANSFORM,			/* pg_transform */
+	OCLASS_VARIABLE				/* pg_variable */
 } ObjectClass;
 
-#define LAST_OCLASS		OCLASS_TRANSFORM
+#define LAST_OCLASS		OCLASS_VARIABLE
 
 /* flag bits for performDeletion/performMultipleDeletions: */
 #define PERFORM_DELETION_INTERNAL			0x0001	/* internal action */
diff --git a/src/include/catalog/namespace.h b/src/include/catalog/namespace.h
index f963d82797..6e072447a5 100644
--- a/src/include/catalog/namespace.h
+++ b/src/include/catalog/namespace.h
@@ -96,6 +96,8 @@ extern Oid	TypenameGetTypid(const char *typname);
 extern Oid	TypenameGetTypidExtended(const char *typname, bool temp_ok);
 extern bool TypeIsVisible(Oid typid);
 
+extern bool VariableIsVisible(Oid varid);
+
 extern FuncCandidateList FuncnameGetCandidates(List *names,
 											   int nargs, List *argnames,
 											   bool expand_variadic,
@@ -164,6 +166,10 @@ extern void SetTempNamespaceState(Oid tempNamespaceId,
 								  Oid tempToastNamespaceId);
 extern void ResetTempTableNamespace(void);
 
+extern List *NamesFromList(List *names);
+extern Oid	LookupVariable(const char *nspname, const char *varname, bool missing_ok);
+extern Oid	IdentifyVariable(List *names, char **attrname, bool lockit, bool *not_unique);
+
 extern OverrideSearchPath *GetOverrideSearchPath(MemoryContext context);
 extern OverrideSearchPath *CopyOverrideSearchPath(OverrideSearchPath *path);
 extern bool OverrideSearchPathMatchesCurrent(OverrideSearchPath *path);
diff --git a/src/include/catalog/pg_default_acl.h b/src/include/catalog/pg_default_acl.h
index 2a79155636..672c571562 100644
--- a/src/include/catalog/pg_default_acl.h
+++ b/src/include/catalog/pg_default_acl.h
@@ -66,6 +66,7 @@ DECLARE_UNIQUE_INDEX_PKEY(pg_default_acl_oid_index, 828, DefaultAclOidIndexId, o
 #define DEFACLOBJ_FUNCTION		'f' /* function */
 #define DEFACLOBJ_TYPE			'T' /* type */
 #define DEFACLOBJ_NAMESPACE		'n' /* namespace */
+#define DEFACLOBJ_VARIABLE		'V' /* variable */
 
 #endif							/* EXPOSE_TO_CLIENT_CODE */
 
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 0859dc81ca..966ed86735 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -6246,6 +6246,9 @@
   proname => 'pg_collation_is_visible', procost => '10', provolatile => 's',
   prorettype => 'bool', proargtypes => 'oid',
   prosrc => 'pg_collation_is_visible' },
+{ oid => '9221', descr => 'is session variable visible in search path?',
+  proname => 'pg_variable_is_visible', procost => '10', provolatile => 's',
+  prorettype => 'bool', proargtypes => 'oid', prosrc => 'pg_variable_is_visible' },
 
 { oid => '2854', descr => 'get OID of current session\'s temp schema, if any',
   proname => 'pg_my_temp_schema', provolatile => 's', proparallel => 'r',
diff --git a/src/include/catalog/pg_variable.h b/src/include/catalog/pg_variable.h
new file mode 100644
index 0000000000..b6716ca8cc
--- /dev/null
+++ b/src/include/catalog/pg_variable.h
@@ -0,0 +1,117 @@
+/*-------------------------------------------------------------------------
+ *
+ * pg_variable.h
+ *	  definition of session variables system catalog (pg_variables)
+ *
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/catalog/pg_variable.h
+ *
+ * NOTES
+ *	  The Catalog.pm module reads this file and derives schema
+ *	  information.
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PG_VARIABLE_H
+#define PG_VARIABLE_H
+
+#include "catalog/genbki.h"
+#include "catalog/objectaddress.h"
+#include "catalog/pg_variable_d.h"
+#include "utils/acl.h"
+
+/* ----------------
+ *		pg_variable definition.  cpp turns this into
+ *		typedef struct FormData_pg_variable
+ * ----------------
+ */
+CATALOG(pg_variable,9222,VariableRelationId)
+{
+	Oid			oid;			/* oid */
+	NameData	varname;		/* variable name */
+	Oid			varnamespace;	/* OID of namespace containing variable class */
+	Oid			vartype;		/* OID of entry in pg_type for variable's type */
+	int32		vartypmod;		/* typmode for variable's type */
+	Oid			varowner;		/* class owner */
+	Oid			varcollation;	/* variable collation */
+	bool		varisnotnull;	/* Don't allow NULL */
+	bool		varisimmutable;	/* Don't allow changes */
+	char		vareoxaction;	/* action on transaction end */
+
+#ifdef CATALOG_VARLEN			/* variable-length fields start here */
+
+	/* list of expression trees for variable default (NULL if none) */
+	pg_node_tree vardefexpr BKI_DEFAULT(_null_);
+
+	aclitem		varacl[1] BKI_DEFAULT(_null_);	/* access permissions */
+
+#endif
+} FormData_pg_variable;
+
+typedef enum VariableEOXActionCodes
+{
+	VARIABLE_EOX_CODE_NOOP = 'n',	/* NOOP */
+	VARIABLE_EOX_CODE_DROP = 'd',	/* ON COMMIT DROP */
+	VARIABLE_EOX_CODE_RESET = 'r',	/* ON COMMIT RESET */
+}			VariableEOXActionCodes;
+
+/* ----------------
+ *		Form_pg_variable corresponds to a pointer to a tuple with
+ *		the format of pg_variable relation.
+ * ----------------
+ */
+typedef FormData_pg_variable *Form_pg_variable;
+
+DECLARE_UNIQUE_INDEX_PKEY(pg_variable_oid_index, 9223, VariableOidIndexId, on pg_variable using btree(oid oid_ops));
+#define VariableObjectIndexId 9223
+
+DECLARE_UNIQUE_INDEX(pg_variable_varname_nsp_index, 9224, VariableNameNspIndexId,  on pg_variable using btree(varname name_ops, varnamespace oid_ops));
+#define VariableNameNspIndexId  9224
+
+typedef struct Variable
+{
+	Oid			oid;
+	char	   *name;
+	Oid			namespace;
+	Oid			typid;
+	int32		typmod;
+	Oid			owner;
+	Oid			collation;
+	bool		is_not_null;
+	bool		is_immutable;
+	VariableEOXAction eoxaction;
+	bool		has_defexpr;
+	Node	   *defexpr;
+	Acl		   *acl;
+} Variable;
+
+/* returns fields from pg_variable table */
+extern char *get_session_variable_name(Oid varid);
+extern void get_session_variable_type_typmod_collid(Oid varid,
+												   Oid *typid,
+												   int32 *typmod,
+												   Oid *collid);
+
+/* returns name of variable based on current search path */
+extern char *session_variable_get_name(Oid varid);
+
+extern void initVariable(Variable *var,
+						 Oid varid,
+						 bool missing_ok,
+						 bool fast_only);
+extern ObjectAddress VariableCreate(const char *varName,
+									Oid varNamespace,
+									Oid varType,
+									int32 varTypmod,
+									Oid varOwner,
+									Oid varCollation,
+									Node *varDefexpr,
+									VariableEOXAction eoxaction,
+									bool is_not_null,
+									bool if_not_exists,
+									bool is_immutable);
+
+#endif							/* PG_VARIABLE_H */
diff --git a/src/include/commands/session_variable.h b/src/include/commands/session_variable.h
new file mode 100644
index 0000000000..c8c3ab2570
--- /dev/null
+++ b/src/include/commands/session_variable.h
@@ -0,0 +1,44 @@
+/*-------------------------------------------------------------------------
+ *
+ * sessionvariable.h
+ *	  prototypes for sessionvariable.c.
+ *
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/commands/session_variable.h
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef SESSIONVARIABLE_H
+#define SESSIONVARIABLE_H
+
+#include "catalog/objectaddress.h"
+#include "catalog/pg_variable.h"
+#include "nodes/params.h"
+#include "nodes/parsenodes.h"
+#include "nodes/plannodes.h"
+#include "tcop/cmdtag.h"
+#include "utils/queryenvironment.h"
+
+extern void ResetSessionVariables(void);
+extern void RemoveSessionVariable(Oid varid);
+extern ObjectAddress DefineSessionVariable(ParseState *pstate, CreateSessionVarStmt * stmt);
+
+extern Datum GetSessionVariable(Oid varid, bool *isNull, Oid expected_typid, bool copy);
+extern Datum CopySessionVariable(Oid varid, bool *isNull, Oid *typid);
+extern void SetSessionVariable(Oid varid, Datum value, bool isNull, Oid typid);
+extern void SetSessionVariableWithSecurityCheck(Oid varid, Datum value, bool isNull, Oid typid);
+
+extern void ExecuteLetStmt(ParseState *pstate, LetStmt *stmt, ParamListInfo params,
+						   QueryEnvironment *queryEnv, QueryCompletion *qc);
+
+extern void RegisterOnCommitDropSessionVariable(Oid varid);
+
+extern void AtPreEOXact_SessionVariable_on_xact_actions(bool isCommit);
+extern void AtEOSubXact_SessionVariable_on_xact_actions(bool isCommit, SubTransactionId mySubid,
+											SubTransactionId parentSubid);
+
+#endif
diff --git a/src/include/executor/execExpr.h b/src/include/executor/execExpr.h
index 56a89ebafb..f59243e558 100644
--- a/src/include/executor/execExpr.h
+++ b/src/include/executor/execExpr.h
@@ -158,6 +158,7 @@ typedef enum ExprEvalOp
 	EEOP_PARAM_EXEC,
 	EEOP_PARAM_EXTERN,
 	EEOP_PARAM_CALLBACK,
+	EEOP_PARAM_VARIABLE,
 
 	/* return CaseTestExpr value */
 	EEOP_CASE_TESTVAL,
@@ -380,6 +381,13 @@ typedef struct ExprEvalStep
 			Oid			paramtype;	/* OID of parameter's datatype */
 		}			param;
 
+		/* for EEOP_PARAM_VARIABLE */
+		struct
+		{
+			Oid			varid;	/* OID of assigned variable */
+			Oid			vartype;	/* OID of parameter's datatype */
+		}			vparam;
+
 		/* for EEOP_PARAM_CALLBACK */
 		struct
 		{
@@ -736,6 +744,8 @@ extern void ExecEvalParamExec(ExprState *state, ExprEvalStep *op,
 							  ExprContext *econtext);
 extern void ExecEvalParamExtern(ExprState *state, ExprEvalStep *op,
 								ExprContext *econtext);
+extern void ExecEvalParamVariable(ExprState *state, ExprEvalStep *op,
+								  ExprContext *econtext);
 extern void ExecEvalSQLValueFunction(ExprState *state, ExprEvalStep *op);
 extern void ExecEvalCurrentOfExpr(ExprState *state, ExprEvalStep *op);
 extern void ExecEvalNextValueExpr(ExprState *state, ExprEvalStep *op);
diff --git a/src/include/executor/execdesc.h b/src/include/executor/execdesc.h
index e79e2c001f..dbf4dc7ea0 100644
--- a/src/include/executor/execdesc.h
+++ b/src/include/executor/execdesc.h
@@ -48,6 +48,10 @@ typedef struct QueryDesc
 	EState	   *estate;			/* executor's query-wide state */
 	PlanState  *planstate;		/* tree of per-plan-node state */
 
+	/* reference to session variables buffer */
+	int			num_session_variables;
+	SessionVariableValue *session_variables;
+
 	/* This field is set by ExecutorRun */
 	bool		already_executed;	/* true if previously executed */
 
diff --git a/src/include/executor/svariableReceiver.h b/src/include/executor/svariableReceiver.h
new file mode 100644
index 0000000000..5305936547
--- /dev/null
+++ b/src/include/executor/svariableReceiver.h
@@ -0,0 +1,25 @@
+/*-------------------------------------------------------------------------
+ *
+ * svariableReceiver.h
+ *	  prototypes for svariableReceiver.c
+ *
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/executor/svariableReceiver.h
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef SVARIABLE_RECEIVER_H
+#define SVARIABLE_RECEIVER_H
+
+#include "tcop/dest.h"
+
+
+extern DestReceiver *CreateVariableDestReceiver(void);
+
+extern void SetVariableDestReceiverParams(DestReceiver *self, Oid varid);
+
+#endif							/* SVARIABLE_RECEIVER_H */
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 4ea8735dd8..e506802eaa 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -548,6 +548,18 @@ typedef struct AsyncRequest
 								 * tuples) */
 } AsyncRequest;
 
+/* ----------------
+ * SessionVariableValue
+ * ----------------
+ */
+typedef struct SessionVariableValue
+{
+	Oid			varid;
+	Oid			typid;
+	bool		isnull;
+	Datum		value;
+}			SessionVariableValue;
+
 /* ----------------
  *	  EState information
  *
@@ -599,6 +611,13 @@ typedef struct EState
 	ParamListInfo es_param_list_info;	/* values of external params */
 	ParamExecData *es_param_exec_vals;	/* values of internal params */
 
+	/* Variables info: */
+	/* number of used session variables */
+	int			es_num_session_variables;
+
+	/* array of copied values of session variables */
+	SessionVariableValue *es_session_variables;
+
 	QueryEnvironment *es_queryEnv;	/* query environment */
 
 	/* Other working state: */
diff --git a/src/include/nodes/nodes.h b/src/include/nodes/nodes.h
index da35f2c272..002b6e4969 100644
--- a/src/include/nodes/nodes.h
+++ b/src/include/nodes/nodes.h
@@ -356,6 +356,7 @@ typedef enum NodeTag
 	T_CreateTableAsStmt,
 	T_CreateSeqStmt,
 	T_AlterSeqStmt,
+	T_CreateSessionVarStmt,
 	T_VariableSetStmt,
 	T_VariableShowStmt,
 	T_DiscardStmt,
@@ -429,6 +430,7 @@ typedef enum NodeTag
 	T_AlterCollationStmt,
 	T_CallStmt,
 	T_AlterStatsStmt,
+	T_LetStmt,
 
 	/*
 	 * TAGS FOR PARSE TREE NODES (parsenodes.h)
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index 3e9bdc781f..6721cbea7c 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -92,7 +92,9 @@ typedef uint32 AclMode;			/* a bitmask of privilege bits */
 #define ACL_CREATE		(1<<9)	/* for namespaces and databases */
 #define ACL_CREATE_TEMP (1<<10) /* for databases */
 #define ACL_CONNECT		(1<<11) /* for databases */
-#define N_ACL_RIGHTS	12		/* 1 plus the last 1<<x */
+#define ACL_READ		(1<<12) /* for variables */
+#define ACL_WRITE		(1<<13) /* for variables */
+#define N_ACL_RIGHTS	14		/* 1 plus the last 1<<x */
 #define ACL_NO_RIGHTS	0
 /* Currently, SELECT ... FOR [KEY] UPDATE/SHARE requires UPDATE privileges */
 #define ACL_SELECT_FOR_UPDATE	ACL_UPDATE
@@ -129,7 +131,7 @@ typedef struct Query
 
 	int			resultRelation; /* rtable index of target relation for
 								 * INSERT/UPDATE/DELETE; 0 for SELECT */
-
+	Oid			resultVariable;	/* target variable of LET statement */
 	bool		hasAggs;		/* has aggregates in tlist or havingQual */
 	bool		hasWindowFuncs; /* has window functions in tlist */
 	bool		hasTargetSRFs;	/* has set-returning functions in tlist */
@@ -139,6 +141,7 @@ typedef struct Query
 	bool		hasModifyingCTE;	/* has INSERT/UPDATE/DELETE in WITH */
 	bool		hasForUpdate;	/* FOR [KEY] UPDATE/SHARE was specified */
 	bool		hasRowSecurity; /* rewriter has applied some RLS policy */
+	bool		hasSessionVariables; /* uses session variables */
 
 	bool		isReturn;		/* is a RETURN statement */
 
@@ -1628,6 +1631,21 @@ typedef struct UpdateStmt
 	WithClause *withClause;		/* WITH clause */
 } UpdateStmt;
 
+/* ----------------------
+ *		Let Statement
+ * ----------------------
+ */
+typedef struct LetStmt
+{
+	NodeTag		type;
+	List	   *target;			/* target variable */
+	Node	   *query;			/* source expression */
+	bool		set_default;	/* true, when set to DEFAULt is wanted */
+	bool		plpgsql_mode;	/* true, when command will be executed (parsed)
+								 * by plpgsql runtime */
+	int			location;
+}			LetStmt;
+
 /* ----------------------
  *		Select Statement
  *
@@ -1837,6 +1855,7 @@ typedef enum ObjectType
 	OBJECT_TSTEMPLATE,
 	OBJECT_TYPE,
 	OBJECT_USER_MAPPING,
+	OBJECT_VARIABLE,
 	OBJECT_VIEW
 } ObjectType;
 
@@ -2673,6 +2692,23 @@ typedef struct AlterSeqStmt
 	bool		missing_ok;		/* skip error if a role is missing? */
 } AlterSeqStmt;
 
+/* ----------------------
+ *		{Create|Alter} VARIABLE Statement
+ * ----------------------
+ */
+typedef struct CreateSessionVarStmt
+{
+	NodeTag		type;
+	RangeVar   *variable;		/* the variable to create */
+	TypeName   *typeName;		/* the type of variable */
+	CollateClause *collClause;
+	Node	   *defexpr;		/* default expression */
+	VariableEOXAction eoxaction;	/* on commit action */
+	bool		if_not_exists;	/* do nothing if it already exists */
+	bool		is_not_null;	/* Disallow nulls */
+	bool		is_immutable;	/* Don't allow changes */
+}			CreateSessionVarStmt;
+
 /* ----------------------
  *		Create {Aggregate|Operator|Type} Statement
  * ----------------------
@@ -3446,7 +3482,8 @@ typedef enum DiscardMode
 	DISCARD_ALL,
 	DISCARD_PLANS,
 	DISCARD_SEQUENCES,
-	DISCARD_TEMP
+	DISCARD_TEMP,
+	DISCARD_VARIABLES
 } DiscardMode;
 
 typedef struct DiscardStmt
diff --git a/src/include/nodes/pathnodes.h b/src/include/nodes/pathnodes.h
index 1f3845b3fe..5a54e2d42d 100644
--- a/src/include/nodes/pathnodes.h
+++ b/src/include/nodes/pathnodes.h
@@ -130,6 +130,8 @@ typedef struct PlannerGlobal
 	char		maxParallelHazard;	/* worst PROPARALLEL hazard level */
 
 	PartitionDirectory partition_directory; /* partition descriptors */
+
+	List	   *sessionVariables;	/* list of used session variables */
 } PlannerGlobal;
 
 /* macro for fetching the Plan associated with a SubPlan node */
@@ -350,6 +352,7 @@ struct PlannerInfo
 										 * pseudoconstant = true */
 	bool		hasAlternativeSubPlans; /* true if we've made any of those */
 	bool		hasRecursion;	/* true if planning a recursive WITH item */
+	bool		hasSessionVariables;	/* true if session variables were used */
 
 	/*
 	 * Information about aggregates. Filled by preprocess_aggrefs().
diff --git a/src/include/nodes/plannodes.h b/src/include/nodes/plannodes.h
index 0b518ce6b2..54b1270ab9 100644
--- a/src/include/nodes/plannodes.h
+++ b/src/include/nodes/plannodes.h
@@ -43,7 +43,7 @@ typedef struct PlannedStmt
 {
 	NodeTag		type;
 
-	CmdType		commandType;	/* select|insert|update|delete|utility */
+	CmdType		commandType;	/* select|let|insert|update|delete|utility */
 
 	uint64		queryId;		/* query identifier (copied from Query) */
 
@@ -85,6 +85,8 @@ typedef struct PlannedStmt
 
 	Node	   *utilityStmt;	/* non-null if this is utility stmt */
 
+	List	   *sessionVariables;	/* list of OIDs for PARAM_VARIABLE Params */
+
 	/* statement location in source string (copied from Query) */
 	int			stmt_location;	/* start location, or -1 if unknown */
 	int			stmt_len;		/* length in bytes; 0 means "rest of string" */
diff --git a/src/include/nodes/primnodes.h b/src/include/nodes/primnodes.h
index dab5c4ff5d..4fa6b089fa 100644
--- a/src/include/nodes/primnodes.h
+++ b/src/include/nodes/primnodes.h
@@ -43,7 +43,10 @@ typedef struct Alias
 	List	   *colnames;		/* optional list of column aliases */
 } Alias;
 
-/* What to do at commit time for temporary relations */
+/*
+ * What to do at commit time for temporary relations or
+ * persistent/temporary variable.
+ */
 typedef enum OnCommitAction
 {
 	ONCOMMIT_NOOP,				/* No ON COMMIT clause (do nothing) */
@@ -52,6 +55,13 @@ typedef enum OnCommitAction
 	ONCOMMIT_DROP				/* ON COMMIT DROP */
 } OnCommitAction;
 
+typedef enum VariableEOXAction
+{
+	VARIABLE_EOX_NOOP,			/* Do nothing */
+	VARIABLE_EOX_DROP,			/* ON TRANSACTION END DROP */
+	VARIABLE_EOX_RESET,			/* ON TRANSACTION END RESET */
+}			VariableEOXAction;
+
 /*
  * RangeVar - range variable, used in FROM clauses
  *
@@ -252,13 +262,17 @@ typedef struct Const
  *				of the `paramid' field contain the SubLink's subLinkId, and
  *				the low-order 16 bits contain the column number.  (This type
  *				of Param is also converted to PARAM_EXEC during planning.)
+ *
+ *		PARAM_VARIABLE:  The parameter is an access to session variable
+ *				paramid holds varid.
  */
 typedef enum ParamKind
 {
 	PARAM_EXTERN,
 	PARAM_EXEC,
 	PARAM_SUBLINK,
-	PARAM_MULTIEXPR
+	PARAM_MULTIEXPR,
+	PARAM_VARIABLE
 } ParamKind;
 
 typedef struct Param
@@ -269,6 +283,7 @@ typedef struct Param
 	Oid			paramtype;		/* pg_type OID of parameter's datatype */
 	int32		paramtypmod;	/* typmod value, if known */
 	Oid			paramcollid;	/* OID of collation, or InvalidOid if none */
+	Oid			paramvarid;		/* OID of session variable if it is used */
 	int			location;		/* token location, or -1 if unknown */
 } Param;
 
diff --git a/src/include/optimizer/planmain.h b/src/include/optimizer/planmain.h
index 54a0d4c188..912313a5df 100644
--- a/src/include/optimizer/planmain.h
+++ b/src/include/optimizer/planmain.h
@@ -116,4 +116,6 @@ extern void record_plan_function_dependency(PlannerInfo *root, Oid funcid);
 extern void record_plan_type_dependency(PlannerInfo *root, Oid typid);
 extern bool extract_query_dependencies_walker(Node *node, PlannerInfo *root);
 
+extern void pull_up_has_session_variables(PlannerInfo *root);
+
 #endif							/* PLANMAIN_H */
diff --git a/src/include/parser/kwlist.h b/src/include/parser/kwlist.h
index bcef7eed2f..3fe47d5acc 100644
--- a/src/include/parser/kwlist.h
+++ b/src/include/parser/kwlist.h
@@ -237,6 +237,7 @@ PG_KEYWORD("leading", LEADING, RESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("leakproof", LEAKPROOF, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("least", LEAST, COL_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("left", LEFT, TYPE_FUNC_NAME_KEYWORD, BARE_LABEL)
+PG_KEYWORD("let", LET, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("level", LEVEL, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("like", LIKE, TYPE_FUNC_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("limit", LIMIT, RESERVED_KEYWORD, AS_LABEL)
@@ -450,6 +451,8 @@ PG_KEYWORD("validator", VALIDATOR, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("value", VALUE_P, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("values", VALUES, COL_NAME_KEYWORD, BARE_LABEL)
 PG_KEYWORD("varchar", VARCHAR, COL_NAME_KEYWORD, BARE_LABEL)
+PG_KEYWORD("variable", VARIABLE, UNRESERVED_KEYWORD, BARE_LABEL)
+PG_KEYWORD("variables", VARIABLES, UNRESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("variadic", VARIADIC, RESERVED_KEYWORD, BARE_LABEL)
 PG_KEYWORD("varying", VARYING, UNRESERVED_KEYWORD, AS_LABEL)
 PG_KEYWORD("verbose", VERBOSE, TYPE_FUNC_NAME_KEYWORD, BARE_LABEL)
diff --git a/src/include/parser/parse_expr.h b/src/include/parser/parse_expr.h
index 308e84edda..9734b86872 100644
--- a/src/include/parser/parse_expr.h
+++ b/src/include/parser/parse_expr.h
@@ -17,6 +17,7 @@
 
 /* GUC parameters */
 extern bool Transform_null_equals;
+extern bool session_variables_ambiguity_warning;
 
 extern Node *transformExpr(ParseState *pstate, Node *expr, ParseExprKind exprKind);
 
diff --git a/src/include/parser/parse_node.h b/src/include/parser/parse_node.h
index 8c859d0d0e..e5687814fd 100644
--- a/src/include/parser/parse_node.h
+++ b/src/include/parser/parse_node.h
@@ -80,6 +80,8 @@ typedef enum ParseExprKind
 	EXPR_KIND_COPY_WHERE,		/* WHERE condition in COPY FROM */
 	EXPR_KIND_GENERATED_COLUMN, /* generation expression for a column */
 	EXPR_KIND_CYCLE_MARK,		/* cycle mark value */
+	EXPR_KIND_VARIABLE_DEFAULT, /* default value for session variable */
+	EXPR_KIND_LET_TARGET		/* LET assignment (should be same like UPDATE) */
 } ParseExprKind;
 
 
@@ -210,6 +212,7 @@ struct ParseState
 	bool		p_hasTargetSRFs;
 	bool		p_hasSubLinks;
 	bool		p_hasModifyingCTE;
+	bool		p_hasSessionVariables;
 
 	Node	   *p_last_srf;		/* most recent set-returning func/op found */
 
diff --git a/src/include/parser/parser.h b/src/include/parser/parser.h
index 6aac0e096a..8e18ab172e 100644
--- a/src/include/parser/parser.h
+++ b/src/include/parser/parser.h
@@ -33,6 +33,9 @@
  * RAW_PARSE_PLPGSQL_ASSIGNn: parse a PL/pgSQL assignment statement,
  * and return a one-element List containing a RawStmt node.  "n"
  * gives the number of dotted names comprising the target ColumnRef.
+ *
+ * RAW_PARSE_PLPGSQL_LET: parse a LET statement, and return a
+ * one-element List containing a RawStmt node.
  */
 typedef enum
 {
@@ -41,7 +44,8 @@ typedef enum
 	RAW_PARSE_PLPGSQL_EXPR,
 	RAW_PARSE_PLPGSQL_ASSIGN1,
 	RAW_PARSE_PLPGSQL_ASSIGN2,
-	RAW_PARSE_PLPGSQL_ASSIGN3
+	RAW_PARSE_PLPGSQL_ASSIGN3,
+	RAW_PARSE_PLPGSQL_LET
 } RawParseMode;
 
 /* Values for the backslash_quote GUC */
diff --git a/src/include/tcop/cmdtaglist.h b/src/include/tcop/cmdtaglist.h
index 4bc7ddf410..7d890cc1ac 100644
--- a/src/include/tcop/cmdtaglist.h
+++ b/src/include/tcop/cmdtaglist.h
@@ -68,6 +68,7 @@ PG_CMDTAG(CMDTAG_ALTER_TRANSFORM, "ALTER TRANSFORM", true, false, false)
 PG_CMDTAG(CMDTAG_ALTER_TRIGGER, "ALTER TRIGGER", true, false, false)
 PG_CMDTAG(CMDTAG_ALTER_TYPE, "ALTER TYPE", true, true, false)
 PG_CMDTAG(CMDTAG_ALTER_USER_MAPPING, "ALTER USER MAPPING", true, false, false)
+PG_CMDTAG(CMDTAG_ALTER_VARIABLE, "ALTER VARIABLE", true, false, false)
 PG_CMDTAG(CMDTAG_ALTER_VIEW, "ALTER VIEW", true, false, false)
 PG_CMDTAG(CMDTAG_ANALYZE, "ANALYZE", false, false, false)
 PG_CMDTAG(CMDTAG_BEGIN, "BEGIN", false, false, false)
@@ -123,6 +124,7 @@ PG_CMDTAG(CMDTAG_CREATE_TRANSFORM, "CREATE TRANSFORM", true, false, false)
 PG_CMDTAG(CMDTAG_CREATE_TRIGGER, "CREATE TRIGGER", true, false, false)
 PG_CMDTAG(CMDTAG_CREATE_TYPE, "CREATE TYPE", true, false, false)
 PG_CMDTAG(CMDTAG_CREATE_USER_MAPPING, "CREATE USER MAPPING", true, false, false)
+PG_CMDTAG(CMDTAG_CREATE_VARIABLE, "CREATE VARIABLE", true, false, false)
 PG_CMDTAG(CMDTAG_CREATE_VIEW, "CREATE VIEW", true, false, false)
 PG_CMDTAG(CMDTAG_DEALLOCATE, "DEALLOCATE", false, false, false)
 PG_CMDTAG(CMDTAG_DEALLOCATE_ALL, "DEALLOCATE ALL", false, false, false)
@@ -133,6 +135,7 @@ PG_CMDTAG(CMDTAG_DISCARD_ALL, "DISCARD ALL", false, false, false)
 PG_CMDTAG(CMDTAG_DISCARD_PLANS, "DISCARD PLANS", false, false, false)
 PG_CMDTAG(CMDTAG_DISCARD_SEQUENCES, "DISCARD SEQUENCES", false, false, false)
 PG_CMDTAG(CMDTAG_DISCARD_TEMP, "DISCARD TEMP", false, false, false)
+PG_CMDTAG(CMDTAG_DISCARD_VARIABLES, "DISCARD VARIABLES", false, false, false)
 PG_CMDTAG(CMDTAG_DO, "DO", false, false, false)
 PG_CMDTAG(CMDTAG_DROP_ACCESS_METHOD, "DROP ACCESS METHOD", true, false, false)
 PG_CMDTAG(CMDTAG_DROP_AGGREGATE, "DROP AGGREGATE", true, false, false)
@@ -175,6 +178,7 @@ PG_CMDTAG(CMDTAG_DROP_TRANSFORM, "DROP TRANSFORM", true, false, false)
 PG_CMDTAG(CMDTAG_DROP_TRIGGER, "DROP TRIGGER", true, false, false)
 PG_CMDTAG(CMDTAG_DROP_TYPE, "DROP TYPE", true, false, false)
 PG_CMDTAG(CMDTAG_DROP_USER_MAPPING, "DROP USER MAPPING", true, false, false)
+PG_CMDTAG(CMDTAG_DROP_VARIABLE, "DROP VARIABLE", true, false, false)
 PG_CMDTAG(CMDTAG_DROP_VIEW, "DROP VIEW", true, false, false)
 PG_CMDTAG(CMDTAG_EXECUTE, "EXECUTE", false, false, false)
 PG_CMDTAG(CMDTAG_EXPLAIN, "EXPLAIN", false, false, false)
@@ -183,6 +187,7 @@ PG_CMDTAG(CMDTAG_GRANT, "GRANT", true, false, false)
 PG_CMDTAG(CMDTAG_GRANT_ROLE, "GRANT ROLE", false, false, false)
 PG_CMDTAG(CMDTAG_IMPORT_FOREIGN_SCHEMA, "IMPORT FOREIGN SCHEMA", true, false, false)
 PG_CMDTAG(CMDTAG_INSERT, "INSERT", false, false, true)
+PG_CMDTAG(CMDTAG_LET, "LET", false, false, false)
 PG_CMDTAG(CMDTAG_LISTEN, "LISTEN", false, false, false)
 PG_CMDTAG(CMDTAG_LOAD, "LOAD", false, false, false)
 PG_CMDTAG(CMDTAG_LOCK_TABLE, "LOCK TABLE", false, false, false)
diff --git a/src/include/tcop/dest.h b/src/include/tcop/dest.h
index 3c3eabae67..7f18e07915 100644
--- a/src/include/tcop/dest.h
+++ b/src/include/tcop/dest.h
@@ -97,7 +97,8 @@ typedef enum
 	DestCopyOut,				/* results sent to COPY TO code */
 	DestSQLFunction,			/* results sent to SQL-language func mgr */
 	DestTransientRel,			/* results sent to transient relation */
-	DestTupleQueue				/* results sent to tuple queue */
+	DestTupleQueue,				/* results sent to tuple queue */
+	DestVariable				/* results sents to session variable */
 } CommandDest;
 
 /* ----------------
diff --git a/src/include/utils/acl.h b/src/include/utils/acl.h
index 1ce4c5556e..955cebec88 100644
--- a/src/include/utils/acl.h
+++ b/src/include/utils/acl.h
@@ -146,9 +146,11 @@ typedef struct ArrayType Acl;
 #define ACL_CREATE_CHR			'C'
 #define ACL_CREATE_TEMP_CHR		'T'
 #define ACL_CONNECT_CHR			'c'
+#define ACL_READ_CHR			'S' /* 'R' is occupated by old RULE priv */
+#define ACL_WRITE_CHR			'W'
 
 /* string holding all privilege code chars, in order by bitmask position */
-#define ACL_ALL_RIGHTS_STR	"arwdDxtXUCTc"
+#define ACL_ALL_RIGHTS_STR	"arwdDxtXUCTcSW"
 
 /*
  * Bitmasks defining "all rights" for each supported object type
@@ -165,6 +167,7 @@ typedef struct ArrayType Acl;
 #define ACL_ALL_RIGHTS_SCHEMA		(ACL_USAGE|ACL_CREATE)
 #define ACL_ALL_RIGHTS_TABLESPACE	(ACL_CREATE)
 #define ACL_ALL_RIGHTS_TYPE			(ACL_USAGE)
+#define ACL_ALL_RIGHTS_VARIABLE		(ACL_READ|ACL_WRITE)
 
 /* operation codes for pg_*_aclmask */
 typedef enum
@@ -259,7 +262,8 @@ extern AclMode pg_foreign_server_aclmask(Oid srv_oid, Oid roleid,
 										 AclMode mask, AclMaskHow how);
 extern AclMode pg_type_aclmask(Oid type_oid, Oid roleid,
 							   AclMode mask, AclMaskHow how);
-
+extern AclMode pg_variable_aclmask(Oid var_oid, Oid roleid,
+								   AclMode mask, AclMaskHow how);
 extern AclResult pg_attribute_aclcheck(Oid table_oid, AttrNumber attnum,
 									   Oid roleid, AclMode mode);
 extern AclResult pg_attribute_aclcheck_ext(Oid table_oid, AttrNumber attnum,
@@ -280,6 +284,7 @@ extern AclResult pg_tablespace_aclcheck(Oid spc_oid, Oid roleid, AclMode mode);
 extern AclResult pg_foreign_data_wrapper_aclcheck(Oid fdw_oid, Oid roleid, AclMode mode);
 extern AclResult pg_foreign_server_aclcheck(Oid srv_oid, Oid roleid, AclMode mode);
 extern AclResult pg_type_aclcheck(Oid type_oid, Oid roleid, AclMode mode);
+extern AclResult pg_variable_aclcheck(Oid type_oid, Oid roleid, AclMode mode);
 
 extern void aclcheck_error(AclResult aclerr, ObjectType objtype,
 						   const char *objectname);
@@ -316,6 +321,7 @@ extern bool pg_extension_ownercheck(Oid ext_oid, Oid roleid);
 extern bool pg_publication_ownercheck(Oid pub_oid, Oid roleid);
 extern bool pg_subscription_ownercheck(Oid sub_oid, Oid roleid);
 extern bool pg_statistics_object_ownercheck(Oid stat_oid, Oid roleid);
+extern bool pg_variable_ownercheck(Oid stat_oid, Oid roleid);
 extern bool has_createrole_privilege(Oid roleid);
 extern bool has_bypassrls_privilege(Oid roleid);
 
diff --git a/src/include/utils/lsyscache.h b/src/include/utils/lsyscache.h
index b8dd27d4a9..974bb27e1e 100644
--- a/src/include/utils/lsyscache.h
+++ b/src/include/utils/lsyscache.h
@@ -132,6 +132,7 @@ extern char get_func_prokind(Oid funcid);
 extern bool get_func_leakproof(Oid funcid);
 extern RegProcedure get_func_support(Oid funcid);
 extern Oid	get_relname_relid(const char *relname, Oid relnamespace);
+extern Oid	get_varname_varid(const char *varname, Oid varnamespace);
 extern char *get_rel_name(Oid relid);
 extern Oid	get_rel_namespace(Oid relid);
 extern Oid	get_rel_type_id(Oid relid);
diff --git a/src/include/utils/syscache.h b/src/include/utils/syscache.h
index 9c1a76e8bb..421576cab8 100644
--- a/src/include/utils/syscache.h
+++ b/src/include/utils/syscache.h
@@ -111,9 +111,11 @@ enum SysCacheIdentifier
 	TYPENAMENSP,
 	TYPEOID,
 	USERMAPPINGOID,
-	USERMAPPINGUSERSERVER
+	USERMAPPINGUSERSERVER,
+	VARIABLENAMENSP,
+	VARIABLEOID
 
-#define SysCacheSize (USERMAPPINGUSERSERVER + 1)
+#define SysCacheSize (VARIABLEOID + 1)
 };
 
 extern void InitCatalogCache(void);
diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index 70c4a75295..2941d45e3e 100644
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/src/pl/plpgsql/src/pl_exec.c
@@ -24,6 +24,7 @@
 #include "catalog/pg_proc.h"
 #include "catalog/pg_type.h"
 #include "commands/defrem.h"
+#include "commands/session_variable.h"
 #include "executor/execExpr.h"
 #include "executor/spi.h"
 #include "executor/tstoreReceiver.h"
@@ -318,6 +319,8 @@ static int	exec_stmt_commit(PLpgSQL_execstate *estate,
 							 PLpgSQL_stmt_commit *stmt);
 static int	exec_stmt_rollback(PLpgSQL_execstate *estate,
 							   PLpgSQL_stmt_rollback *stmt);
+static int	exec_stmt_let(PLpgSQL_execstate *estate,
+						  PLpgSQL_stmt_let *let);
 
 static void plpgsql_estate_setup(PLpgSQL_execstate *estate,
 								 PLpgSQL_function *func,
@@ -2103,6 +2106,10 @@ exec_stmts(PLpgSQL_execstate *estate, List *stmts)
 				rc = exec_stmt_rollback(estate, (PLpgSQL_stmt_rollback *) stmt);
 				break;
 
+			case PLPGSQL_STMT_LET:
+				rc = exec_stmt_let(estate, (PLpgSQL_stmt_let *) stmt);
+				break;
+
 			default:
 				/* point err_stmt to parent, since this one seems corrupt */
 				estate->err_stmt = save_estmt;
@@ -4950,6 +4957,54 @@ exec_stmt_rollback(PLpgSQL_execstate *estate, PLpgSQL_stmt_rollback *stmt)
 	return PLPGSQL_RC_OK;
 }
 
+/* ----------
+ * exec_stmt_let			Evaluate an expression and
+ *					put the result into a session variable.
+ * ----------
+ */
+static int
+exec_stmt_let(PLpgSQL_execstate *estate, PLpgSQL_stmt_let *stmt)
+{
+	bool		isNull;
+	Oid			valtype;
+	int32		valtypmod;
+	Datum		value;
+	Oid			varid;
+
+	List	   *plansources;
+	CachedPlanSource *plansource;
+
+	value = exec_eval_expr(estate,
+						   stmt->expr,
+						   &isNull,
+						   &valtype,
+						   &valtypmod);
+
+	/*
+	 * Oid of target session variable is stored in Query structure.
+	 * It is safer to read this value directly from the plan than to
+	 * hold this value in the plpgsql context, because it's not necessary
+	 * to handle invalidation of the cached value. Next operations
+	 * are read only without any allocations, so we can expect that
+	 * retrieving varid from Query should be fast.
+	 */
+	plansources = SPI_plan_get_plan_sources(stmt->expr->plan);
+	if (list_length(plansources) != 1)
+		elog(ERROR, "unexpected length of plansources of query for LET statement");
+
+	plansource = (CachedPlanSource *) linitial(plansources);
+	if (list_length(plansource->query_list) != 1)
+		elog(ERROR, "unexpected length of plansource of query for LET statement");
+
+	varid = linitial_node(Query, plansource->query_list)->resultVariable;
+	if (!OidIsValid(varid))
+		elog(ERROR, "oid of target session variable is not valid");
+
+	SetSessionVariableWithSecurityCheck(varid, value, isNull, valtype);
+
+	return PLPGSQL_RC_OK;
+}
+
 /* ----------
  * exec_assign_expr			Put an expression's result into a variable.
  * ----------
diff --git a/src/pl/plpgsql/src/pl_funcs.c b/src/pl/plpgsql/src/pl_funcs.c
index 93d9cef06b..8bb7d18e2b 100644
--- a/src/pl/plpgsql/src/pl_funcs.c
+++ b/src/pl/plpgsql/src/pl_funcs.c
@@ -288,6 +288,8 @@ plpgsql_stmt_typename(PLpgSQL_stmt *stmt)
 			return "COMMIT";
 		case PLPGSQL_STMT_ROLLBACK:
 			return "ROLLBACK";
+		case PLPGSQL_STMT_LET:
+			return "LET";
 	}
 
 	return "unknown";
@@ -368,6 +370,7 @@ static void free_perform(PLpgSQL_stmt_perform *stmt);
 static void free_call(PLpgSQL_stmt_call *stmt);
 static void free_commit(PLpgSQL_stmt_commit *stmt);
 static void free_rollback(PLpgSQL_stmt_rollback *stmt);
+static void free_let(PLpgSQL_stmt_let *stmt);
 static void free_expr(PLpgSQL_expr *expr);
 
 
@@ -457,6 +460,9 @@ free_stmt(PLpgSQL_stmt *stmt)
 		case PLPGSQL_STMT_ROLLBACK:
 			free_rollback((PLpgSQL_stmt_rollback *) stmt);
 			break;
+		case PLPGSQL_STMT_LET:
+			free_let((PLpgSQL_stmt_let *) stmt);
+			break;
 		default:
 			elog(ERROR, "unrecognized cmd_type: %d", stmt->cmd_type);
 			break;
@@ -711,6 +717,12 @@ free_getdiag(PLpgSQL_stmt_getdiag *stmt)
 {
 }
 
+static void
+free_let(PLpgSQL_stmt_let *stmt)
+{
+	free_expr(stmt->expr);
+}
+
 static void
 free_expr(PLpgSQL_expr *expr)
 {
@@ -813,6 +825,7 @@ static void dump_perform(PLpgSQL_stmt_perform *stmt);
 static void dump_call(PLpgSQL_stmt_call *stmt);
 static void dump_commit(PLpgSQL_stmt_commit *stmt);
 static void dump_rollback(PLpgSQL_stmt_rollback *stmt);
+static void dump_let(PLpgSQL_stmt_let *stmt);
 static void dump_expr(PLpgSQL_expr *expr);
 
 
@@ -912,6 +925,9 @@ dump_stmt(PLpgSQL_stmt *stmt)
 		case PLPGSQL_STMT_ROLLBACK:
 			dump_rollback((PLpgSQL_stmt_rollback *) stmt);
 			break;
+		case PLPGSQL_STMT_LET:
+			dump_let((PLpgSQL_stmt_let *) stmt);
+			break;
 		default:
 			elog(ERROR, "unrecognized cmd_type: %d", stmt->cmd_type);
 			break;
@@ -1588,6 +1604,14 @@ dump_getdiag(PLpgSQL_stmt_getdiag *stmt)
 	printf("\n");
 }
 
+static void
+dump_let(PLpgSQL_stmt_let *stmt)
+{
+	dump_ind();
+	dump_expr(stmt->expr);
+	printf("\n");
+}
+
 static void
 dump_expr(PLpgSQL_expr *expr)
 {
diff --git a/src/pl/plpgsql/src/pl_gram.y b/src/pl/plpgsql/src/pl_gram.y
index 954c2df331..5914ad2b4f 100644
--- a/src/pl/plpgsql/src/pl_gram.y
+++ b/src/pl/plpgsql/src/pl_gram.y
@@ -197,7 +197,7 @@ static	void			check_raise_parameters(PLpgSQL_stmt_raise *stmt);
 %type <stmt>	stmt_return stmt_raise stmt_assert stmt_execsql
 %type <stmt>	stmt_dynexecute stmt_for stmt_perform stmt_call stmt_getdiag
 %type <stmt>	stmt_open stmt_fetch stmt_move stmt_close stmt_null
-%type <stmt>	stmt_commit stmt_rollback
+%type <stmt>	stmt_commit stmt_rollback stmt_let
 %type <stmt>	stmt_case stmt_foreach_a
 
 %type <list>	proc_exceptions
@@ -304,6 +304,7 @@ static	void			check_raise_parameters(PLpgSQL_stmt_raise *stmt);
 %token <keyword>	K_INTO
 %token <keyword>	K_IS
 %token <keyword>	K_LAST
+%token <keyword>	K_LET
 %token <keyword>	K_LOG
 %token <keyword>	K_LOOP
 %token <keyword>	K_MESSAGE
@@ -897,6 +898,8 @@ proc_stmt		: pl_block ';'
 						{ $$ = $1; }
 				| stmt_rollback
 						{ $$ = $1; }
+				| stmt_let
+						{ $$ = $1; }
 				;
 
 stmt_perform	: K_PERFORM
@@ -1013,6 +1016,29 @@ stmt_assign		: T_DATUM
 					}
 				;
 
+stmt_let		: K_LET
+					{
+						PLpgSQL_stmt_let *new;
+						RawParseMode pmode;
+
+						pmode = RAW_PARSE_PLPGSQL_LET;
+
+						new = palloc0(sizeof(PLpgSQL_stmt_let));
+						new->cmd_type = PLPGSQL_STMT_LET;
+						new->lineno   = plpgsql_location_to_lineno(@1);
+						new->stmtid = ++plpgsql_curr_compile->nstatements;
+
+						/* Push back the head name to include it in the stmt */
+						plpgsql_push_back_token(K_LET);
+						new->expr = read_sql_construct(';', 0, 0, ";",
+													   pmode,
+													   false, true, true,
+													   NULL, NULL);
+
+						$$ = (PLpgSQL_stmt *)new;
+					}
+				;
+
 stmt_getdiag	: K_GET getdiag_area_opt K_DIAGNOSTICS getdiag_list ';'
 					{
 						PLpgSQL_stmt_getdiag	 *new;
diff --git a/src/pl/plpgsql/src/pl_reserved_kwlist.h b/src/pl/plpgsql/src/pl_reserved_kwlist.h
index 9043fbddbc..e68b5d5b26 100644
--- a/src/pl/plpgsql/src/pl_reserved_kwlist.h
+++ b/src/pl/plpgsql/src/pl_reserved_kwlist.h
@@ -40,6 +40,7 @@ PG_KEYWORD("from", K_FROM)
 PG_KEYWORD("if", K_IF)
 PG_KEYWORD("in", K_IN)
 PG_KEYWORD("into", K_INTO)
+PG_KEYWORD("let", K_LET)
 PG_KEYWORD("loop", K_LOOP)
 PG_KEYWORD("not", K_NOT)
 PG_KEYWORD("null", K_NULL)
diff --git a/src/pl/plpgsql/src/plpgsql.h b/src/pl/plpgsql/src/plpgsql.h
index 18a4f6c7d3..8f806c88d0 100644
--- a/src/pl/plpgsql/src/plpgsql.h
+++ b/src/pl/plpgsql/src/plpgsql.h
@@ -127,7 +127,8 @@ typedef enum PLpgSQL_stmt_type
 	PLPGSQL_STMT_PERFORM,
 	PLPGSQL_STMT_CALL,
 	PLPGSQL_STMT_COMMIT,
-	PLPGSQL_STMT_ROLLBACK
+	PLPGSQL_STMT_ROLLBACK,
+	PLPGSQL_STMT_LET
 } PLpgSQL_stmt_type;
 
 /*
@@ -519,6 +520,17 @@ typedef struct PLpgSQL_stmt_assign
 	PLpgSQL_expr *expr;
 } PLpgSQL_stmt_assign;
 
+/*
+ * Let statement
+ */
+typedef struct PLpgSQL_stmt_let
+{
+	PLpgSQL_stmt_type cmd_type;
+	int			lineno;
+	unsigned int stmtid;
+	PLpgSQL_expr *expr;
+} PLpgSQL_stmt_let;
+
 /*
  * PERFORM statement
  */
diff --git a/src/test/regress/expected/misc_sanity.out b/src/test/regress/expected/misc_sanity.out
index a57fd142a9..ce9bad7211 100644
--- a/src/test/regress/expected/misc_sanity.out
+++ b/src/test/regress/expected/misc_sanity.out
@@ -60,7 +60,9 @@ ORDER BY 1, 2;
  pg_index                | indpred       | pg_node_tree
  pg_largeobject          | data          | bytea
  pg_largeobject_metadata | lomacl        | aclitem[]
-(11 rows)
+ pg_variable             | varacl        | aclitem[]
+ pg_variable             | vardefexpr    | pg_node_tree
+(13 rows)
 
 -- system catalogs without primary keys
 --
diff --git a/src/test/regress/expected/sanity_check.out b/src/test/regress/expected/sanity_check.out
index 63706a28cc..a71d246128 100644
--- a/src/test/regress/expected/sanity_check.out
+++ b/src/test/regress/expected/sanity_check.out
@@ -165,6 +165,7 @@ pg_ts_parser|t
 pg_ts_template|t
 pg_type|t
 pg_user_mapping|t
+pg_variable|t
 point_tbl|t
 polygon_tbl|t
 quad_box_tbl|t
diff --git a/src/test/regress/expected/session_variables.out b/src/test/regress/expected/session_variables.out
new file mode 100644
index 0000000000..e458c8e53b
--- /dev/null
+++ b/src/test/regress/expected/session_variables.out
@@ -0,0 +1,849 @@
+CREATE SCHEMA svartest;
+SET search_path = svartest;
+CREATE VARIABLE var1 AS integer;
+CREATE TEMP VARIABLE var2 AS text;
+DROP VARIABLE var1, var2;
+-- functional interface
+CREATE VARIABLE var1 AS numeric;
+CREATE ROLE var_test_role;
+GRANT USAGE ON SCHEMA svartest TO var_test_role;
+SET ROLE TO var_test_role;
+-- should fail
+SELECT var1;
+ERROR:  permission denied for session variable var1
+SET ROLE TO DEFAULT;
+GRANT READ ON VARIABLE var1 TO var_test_role;
+SET ROLE TO var_test_role;
+-- should fail
+LET var1 = 10;
+ERROR:  permission denied for session variable var1
+-- should work
+SELECT var1;
+ var1 
+------
+     
+(1 row)
+
+SET ROLE TO DEFAULT;
+GRANT WRITE ON VARIABLE var1 TO var_test_role;
+SET ROLE TO var_test_role;
+-- should work
+LET var1 = 333;
+SET ROLE TO DEFAULT;
+REVOKE ALL ON VARIABLE var1 FROM var_test_role;
+CREATE OR REPLACE FUNCTION secure_var()
+RETURNS int AS $$
+  SELECT svartest.var1::int;
+$$ LANGUAGE sql SECURITY DEFINER;
+SELECT secure_var();
+ secure_var 
+------------
+        333
+(1 row)
+
+SET ROLE TO var_test_role;
+-- should fail
+SELECT svartest.var1;
+ERROR:  permission denied for session variable var1
+-- should work;
+SELECT secure_var();
+ secure_var 
+------------
+        333
+(1 row)
+
+SET ROLE TO DEFAULT;
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM generate_series(1,100) g(v) WHERE v = var1;
+                  QUERY PLAN                   
+-----------------------------------------------
+ Function Scan on pg_catalog.generate_series g
+   Output: v
+   Function Call: generate_series(1, 100)
+   Filter: ((g.v)::numeric = var1)
+(4 rows)
+
+CREATE VIEW schema_var_view AS SELECT var1;
+SELECT * FROM schema_var_view;
+ var1 
+------
+  333
+(1 row)
+
+\c -
+SET search_path = svartest;
+-- should work still, but var will be empty
+SELECT * FROM schema_var_view;
+ var1 
+------
+     
+(1 row)
+
+LET var1 = pi();
+SELECT var1;
+       var1       
+------------------
+ 3.14159265358979
+(1 row)
+
+-- we can see execution plan of LET statement
+EXPLAIN (VERBOSE, COSTS OFF) LET var1 = pi();
+         QUERY PLAN         
+----------------------------
+ SET SESSION VARIABLE
+ Result
+   Output: 3.14159265358979
+(3 rows)
+
+SELECT var1;
+       var1       
+------------------
+ 3.14159265358979
+(1 row)
+
+CREATE VARIABLE var3 AS int;
+CREATE OR REPLACE FUNCTION inc(int)
+RETURNS int AS $$
+BEGIN
+  LET svartest.var3 = COALESCE(svartest.var3 + $1, $1);
+  RETURN var3;
+END;
+$$ LANGUAGE plpgsql;
+SELECT inc(1);
+ inc 
+-----
+   1
+(1 row)
+
+SELECT inc(1);
+ inc 
+-----
+   2
+(1 row)
+
+SELECT inc(1);
+ inc 
+-----
+   3
+(1 row)
+
+SELECT inc(1) FROM generate_series(1,10);
+ inc 
+-----
+   4
+   5
+   6
+   7
+   8
+   9
+  10
+  11
+  12
+  13
+(10 rows)
+
+SET ROLE TO var_test_role;
+-- should fail
+LET var3 = 0;
+ERROR:  permission denied for session variable var3
+SET ROLE TO DEFAULT;
+DROP VIEW schema_var_view;
+DROP VARIABLE var1 CASCADE;
+DROP VARIABLE var3 CASCADE;
+-- composite variables
+CREATE TYPE sv_xyz AS (x int, y int, z numeric(10,2));
+CREATE VARIABLE v1 AS sv_xyz;
+CREATE VARIABLE v2 AS sv_xyz;
+\d v1
+\d v2
+LET v1 = (1,2,3.14);
+LET v2 = (10,20,3.14*10);
+-- should work too - there are prepared casts
+LET v1 = (1,2,3.14);
+SELECT v1;
+     v1     
+------------
+ (1,2,3.14)
+(1 row)
+
+SELECT v2;
+      v2       
+---------------
+ (10,20,31.40)
+(1 row)
+
+SELECT (v1).*;
+ x | y |  z   
+---+---+------
+ 1 | 2 | 3.14
+(1 row)
+
+SELECT (v2).*;
+ x  | y  |   z   
+----+----+-------
+ 10 | 20 | 31.40
+(1 row)
+
+SELECT v1.x + v1.z;
+ ?column? 
+----------
+     4.14
+(1 row)
+
+SELECT v2.x + v2.z;
+ ?column? 
+----------
+    41.40
+(1 row)
+
+-- access to composite fields should be safe too
+-- should fail
+SET ROLE TO var_test_role;
+SELECT v2.x;
+ERROR:  permission denied for session variable v2
+SET ROLE TO DEFAULT;
+DROP VARIABLE v1;
+DROP VARIABLE v2;
+REVOKE USAGE ON SCHEMA svartest FROM var_test_role;
+DROP ROLE var_test_role;
+-- scalar variables should not be in conflict with qualified column
+CREATE VARIABLE varx AS text;
+SELECT varx.relname FROM pg_class varx WHERE varx.relname = 'pg_class';
+ relname  
+----------
+ pg_class
+(1 row)
+
+-- should fail
+SELECT varx.xxx;
+ERROR:  type text is not composite
+-- variables can be updated under RO transaction
+BEGIN;
+SET TRANSACTION READ ONLY;
+LET varx = 'hello';
+COMMIT;
+SELECT varx;
+ varx  
+-------
+ hello
+(1 row)
+
+DROP VARIABLE varx;
+CREATE TYPE t1 AS (a int, b numeric, c text);
+CREATE VARIABLE v1 AS t1;
+LET v1 = (1, pi(), 'hello');
+SELECT v1;
+             v1             
+----------------------------
+ (1,3.14159265358979,hello)
+(1 row)
+
+LET v1.b = 10.2222;
+SELECT v1;
+        v1         
+-------------------
+ (1,10.2222,hello)
+(1 row)
+
+-- should fail
+LET v1.x = 10;
+ERROR:  cannot assign to field "x" of column "x" because there is no such column in data type t1
+LINE 1: LET v1.x = 10;
+            ^
+DROP VARIABLE v1;
+DROP TYPE t1;
+-- arrays are supported
+CREATE VARIABLE va1 AS numeric[];
+LET va1 = ARRAY[1.1,2.1];
+LET va1[1] = 10.1;
+SELECT va1;
+    va1     
+------------
+ {10.1,2.1}
+(1 row)
+
+CREATE TYPE ta2 AS (a numeric, b numeric[]);
+CREATE VARIABLE va2 AS ta2;
+LET va2 = (10.1, ARRAY[0.0, 0.0]);
+LET va2.a = 10.2;
+SELECT va2;
+        va2         
+--------------------
+ (10.2,"{0.0,0.0}")
+(1 row)
+
+LET va2.b[1] = 10.3;
+SELECT va2;
+         va2         
+---------------------
+ (10.2,"{10.3,0.0}")
+(1 row)
+
+DROP VARIABLE va1;
+DROP VARIABLE va2;
+DROP TYPE ta2;
+-- default values
+CREATE VARIABLE v1 AS numeric DEFAULT pi();
+LET v1 = v1 * 2;
+SELECT v1;
+        v1        
+------------------
+ 6.28318530717958
+(1 row)
+
+CREATE TYPE t2 AS (a numeric, b text);
+CREATE VARIABLE v2 AS t2 DEFAULT (NULL, 'Hello');
+LET svartest.v2.a = pi();
+SELECT v2;
+            v2            
+--------------------------
+ (3.14159265358979,Hello)
+(1 row)
+
+-- should fail due dependency
+DROP TYPE t2;
+ERROR:  cannot drop type t2 because other objects depend on it
+DETAIL:  session variable v2 depends on type t2
+HINT:  Use DROP ... CASCADE to drop the dependent objects too.
+-- should be ok
+DROP VARIABLE v1;
+DROP VARIABLE v2;
+-- tests of alters
+CREATE SCHEMA var_schema1;
+CREATE SCHEMA var_schema2;
+CREATE VARIABLE var_schema1.var1 AS integer;
+LET var_schema1.var1 = 1000;
+SELECT var_schema1.var1;
+ var1 
+------
+ 1000
+(1 row)
+
+ALTER VARIABLE var_schema1.var1 SET SCHEMA var_schema2;
+SELECT var_schema2.var1;
+ var1 
+------
+ 1000
+(1 row)
+
+CREATE ROLE var_test_role;
+ALTER VARIABLE var_schema2.var1 OWNER TO var_test_role;
+SET ROLE TO var_test_role;
+-- should fail, no access to schema var_schema2.var
+SELECT var_schema2.var1;
+ERROR:  permission denied for schema var_schema2
+DROP VARIABLE var_schema2.var1;
+ERROR:  permission denied for schema var_schema2
+SET ROLE TO DEFAULT;
+ALTER VARIABLE var_schema2.var1 SET SCHEMA public;
+SET ROLE TO var_test_role;
+SELECT public.var1;
+ var1 
+------
+ 1000
+(1 row)
+
+ALTER VARIABLE public.var1 RENAME TO var1_renamed;
+SELECT public.var1_renamed;
+ var1_renamed 
+--------------
+         1000
+(1 row)
+
+DROP VARIABLE public.var1_renamed;
+SET ROLE TO DEFAULt;
+-- default rights test
+ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT ALL ON VARIABLES TO var_test_role;
+CREATE VARIABLE public.var2 AS int;
+SET ROLE TO var_test_role;
+-- should be ok
+LET public.var2 = 100;
+SELECT public.var2;
+ var2 
+------
+  100
+(1 row)
+
+SET ROLE TO DEFAULt;
+DROP VARIABLE public.var2;
+DROP OWNED BY var_test_role;
+DROP ROLE var_test_role;
+CREATE VARIABLE xx AS text DEFAULT 'hello';
+SELECT xx, upper(xx);
+  xx   | upper 
+-------+-------
+ hello | HELLO
+(1 row)
+
+LET xx = 'Hi';
+SELECT xx;
+ xx 
+----
+ Hi
+(1 row)
+
+DROP VARIABLE xx;
+-- ON TRANSACTION END RESET tests
+CREATE VARIABLE t1 AS int DEFAULT -1 ON TRANSACTION END RESET;
+BEGIN;
+  SELECT t1;
+ t1 
+----
+ -1
+(1 row)
+
+  LET t1 = 100;
+  SELECT t1;
+ t1  
+-----
+ 100
+(1 row)
+
+COMMIT;
+SELECT t1;
+ t1 
+----
+ -1
+(1 row)
+
+BEGIN;
+  SELECT t1;
+ t1 
+----
+ -1
+(1 row)
+
+  LET t1 = 100;
+  SELECT t1;
+ t1  
+-----
+ 100
+(1 row)
+
+ROLLBACK;
+SELECT t1;
+ t1 
+----
+ -1
+(1 row)
+
+DROP VARIABLE t1;
+CREATE VARIABLE v1 AS int DEFAULT 0;
+CREATE VARIABLE v2 AS text DEFAULT 'none';
+LET v1 = 100;
+LET v2 = 'Hello';
+SELECT v1, v2;
+ v1  |  v2   
+-----+-------
+ 100 | Hello
+(1 row)
+
+LET v1 = DEFAULT;
+LET v2 = DEFAULT;
+SELECT v1, v2;
+ v1 |  v2  
+----+------
+  0 | none
+(1 row)
+
+DROP VARIABLE v1;
+DROP VARIABLE v2;
+-- ON COMMIT DROP tests
+-- should be 0 always
+SELECT count(*) FROM pg_variable;
+ count 
+-------
+     0
+(1 row)
+
+CREATE TEMP VARIABLE g AS int ON COMMIT DROP;
+SELECT count(*) FROM pg_variable;
+ count 
+-------
+     0
+(1 row)
+
+BEGIN;
+  CREATE TEMP VARIABLE g AS int ON COMMIT DROP;
+COMMIT;
+SELECT count(*) FROM pg_variable;
+ count 
+-------
+     0
+(1 row)
+
+BEGIN;
+  CREATE TEMP VARIABLE g AS int ON COMMIT DROP;
+ROLLBACK;
+SELECT count(*) FROM pg_variable;
+ count 
+-------
+     0
+(1 row)
+
+-- test on query with workers
+CREATE TABLE svar_test(a int);
+INSERT INTO svar_test SELECT * FROM generate_series(1,1000000);
+ANALYZE svar_test;
+CREATE VARIABLE zero int;
+LET zero = 0;
+-- parallel workers should be used
+EXPLAIN (costs off) SELECT count(*) FROM svar_test WHERE a%10 = zero;
+                    QUERY PLAN                    
+--------------------------------------------------
+ Finalize Aggregate
+   ->  Gather
+         Workers Planned: 2
+         ->  Partial Aggregate
+               ->  Parallel Seq Scan on svar_test
+                     Filter: ((a % 10) = zero)
+(6 rows)
+
+-- result should be 100000
+SELECT count(*) FROM svar_test WHERE a%10 = zero;
+ count  
+--------
+ 100000
+(1 row)
+
+LET zero = (SELECT count(*) FROM svar_test);
+-- result should be 1000000
+SELECT zero;
+  zero   
+---------
+ 1000000
+(1 row)
+
+-- parallel workers should be used
+EXPLAIN (costs off) LET zero = (SELECT count(*) FROM svar_test);
+                        QUERY PLAN                        
+----------------------------------------------------------
+ SET SESSION VARIABLE
+ Result
+   InitPlan 1 (returns $1)
+     ->  Finalize Aggregate
+           ->  Gather
+                 Workers Planned: 2
+                 ->  Partial Aggregate
+                       ->  Parallel Seq Scan on svar_test
+(8 rows)
+
+DROP TABLE svar_test;
+DROP VARIABLE zero;
+-- use variables in prepared statements
+CREATE VARIABLE v AS numeric;
+LET v = 3.14;
+-- use variables in views
+CREATE VIEW vv AS SELECT COALESCE(v, 0) + 1000 AS result;
+SELECT * FROM vv;
+ result  
+---------
+ 1003.14
+(1 row)
+
+-- start a new session
+\c
+SET search_path to svartest;
+SELECT * FROM vv;
+ result 
+--------
+   1000
+(1 row)
+
+LET v = 3.14;
+SELECT * FROM vv;
+ result  
+---------
+ 1003.14
+(1 row)
+
+-- should fail, dependency
+DROP VARIABLE v;
+ERROR:  cannot drop session variable v because other objects depend on it
+DETAIL:  view vv depends on session variable v
+HINT:  Use DROP ... CASCADE to drop the dependent objects too.
+-- should be ok
+DROP VARIABLE v CASCADE;
+NOTICE:  drop cascades to view vv
+-- other features
+CREATE VARIABLE dt AS integer DEFAULT 0;
+LET dt = 100;
+SELECT dt;
+ dt  
+-----
+ 100
+(1 row)
+
+DISCARD VARIABLES;
+SELECT dt;
+ dt 
+----
+  0
+(1 row)
+
+DROP VARIABLE dt;
+-- NOT NULL
+CREATE VARIABLE v1 AS int NOT NULL;
+CREATE VARIABLE v2 AS int NOT NULL DEFAULT NULL;
+-- should fail
+SELECT v1;
+ERROR:  null value is not allowed for NOT NULL session variable "v1"
+DETAIL:  The session variable was not initialized yet.
+SELECT v2;
+ERROR:  null value is not allowed for NOT NULL session variable "v2"
+LET v1 = NULL;
+ERROR:  null value is not allowed for NOT NULL session variable "v1"
+LET v2 = NULL;
+ERROR:  null value is not allowed for NOT NULL session variable "v2"
+LET v1 = DEFAULT;
+ERROR:  null value is not allowed for NOT NULL session variable "v1"
+LET v2 = DEFAULT;
+ERROR:  null value is not allowed for NOT NULL session variable "v2"
+-- should be ok
+LET v1 = 100;
+LET v2 = 1000;
+SELECT v1, v2;
+ v1  |  v2  
+-----+------
+ 100 | 1000
+(1 row)
+
+DROP VARIABLE v1;
+DROP VARIABLE v2;
+CREATE VARIABLE tv AS int;
+CREATE VARIABLE IF NOT EXISTS tv AS int;
+NOTICE:  session variable "tv" already exists, skipping
+DROP VARIABLE tv;
+CREATE IMMUTABLE VARIABLE iv AS int DEFAULT 100;
+SELECT iv;
+ iv  
+-----
+ 100
+(1 row)
+
+-- should fail;
+LET iv = 10000;
+ERROR:  session variable "iv" is declared IMMUTABLE
+DROP VARIABLE iv;
+-- different order
+CREATE IMMUTABLE VARIABLE iv AS int DEFAULT 100;
+-- should to fail
+LET iv = 10000;
+ERROR:  session variable "iv" is declared IMMUTABLE
+-- should be ok
+SELECT iv;
+ iv  
+-----
+ 100
+(1 row)
+
+DROP VARIABLE iv;
+CREATE IMMUTABLE VARIABLE iv AS int;
+-- should be ok
+LET iv = NULL;
+LET iv = NULL;
+-- should be ok
+LET iv = 10;
+-- should fail
+LET iv = 10;
+ERROR:  session variable "iv" is declared IMMUTABLE
+DROP VARIABLE iv;
+-- create variable inside plpgsql block
+DO $$
+BEGIN
+  CREATE VARIABLE do_test_svar AS date DEFAULT '2000-01-01';
+END;
+$$;
+SELECT do_test_svar;
+ do_test_svar 
+--------------
+ 01-01-2000
+(1 row)
+
+DROP VARIABLE do_test_svar;
+-- should fail
+CREATE IMMUTABLE VARIABLE xx AS int NOT NULL;
+ERROR:  IMMUTABLE NOT NULL variable requires default expression
+-- REASSIGN OWNED test
+CREATE ROLE var_test_role1;
+CREATE ROLE var_test_role2;
+CREATE VARIABLE xxx_var AS int;
+ALTER VARIABLE xxx_var OWNER TO var_test_role1;
+REASSIGN OWNED BY var_test_role1 to var_test_role2;
+SELECT varowner::regrole FROM pg_variable WHERE varname = 'xxx_var';
+    varowner    
+----------------
+ var_test_role2
+(1 row)
+
+DROP OWNED BY var_test_role1;
+DROP ROLE var_test_role1;
+SELECT count(*) FROM pg_variable WHERE varname = 'xxx_var';
+ count 
+-------
+     1
+(1 row)
+
+DROP OWNED BY var_test_role2;
+DROP ROLE var_test_role2;
+SELECT count(*) FROM pg_variable WHERE varname = 'xxx_var';
+ count 
+-------
+     0
+(1 row)
+
+-- creating, dropping temporal variable
+BEGIN;
+CREATE TEMP VARIABLE tempvar AS INT ON COMMIT DROP;
+LET tempvar = 100;
+SAVEPOINT s1;
+DROP VARIABLE tempvar;
+ROLLBACK TO s1;
+SELECT tempvar;
+ tempvar 
+---------
+     100
+(1 row)
+
+COMMIT;
+-- should to fail
+LET tempvar = 100;
+ERROR:  session variable "tempvar" doesn't exist
+LINE 1: LET tempvar = 100;
+            ^
+BEGIN;
+SAVEPOINT s1;
+CREATE TEMP VARIABLE tempvar AS INT ON COMMIT DROP;
+LET tempvar = 100;
+ROLLBACK TO s1;
+COMMIT;
+-- should to fail
+LET tempvar = 100;
+ERROR:  session variable "tempvar" doesn't exist
+LINE 1: LET tempvar = 100;
+            ^
+CREATE VARIABLE var1 AS int;
+LET var1 = 100;
+BEGIN;
+DROP VARIABLE var1;
+ROLLBACK;
+SELECT var1;
+ var1 
+------
+  100
+(1 row)
+
+DROP VARIABLE var1;
+CREATE VARIABLE var1 AS int DEFAULT 100;
+COMMENT ON VARIABLE var1 IS 'some variable comment';
+SELECT pg_catalog.obj_description(oid, 'pg_variable') FROM pg_variable WHERE varname = 'var1';
+    obj_description    
+-----------------------
+ some variable comment
+(1 row)
+
+DROP VARIABLE var1;
+CREATE TABLE xxtab(avar int);
+CREATE TYPE xxtype AS (avar int);
+CREATE VARIABLE xxtab AS xxtype;
+INSERT INTO xxtab VALUES(10);
+-- it is ambiguous, but columns are preferred
+SELECT xxtab.avar FROM xxtab;
+ avar 
+------
+   10
+(1 row)
+
+SET session_variables_ambiguity_warning TO on;
+SELECT xxtab.avar FROM xxtab;
+WARNING:  session variable "xxtab.avar" is shadowed
+LINE 1: SELECT xxtab.avar FROM xxtab;
+               ^
+DETAIL:  Session variables can be shadowed by columns, routine's variables and routine's arguments with same name.
+ avar 
+------
+   10
+(1 row)
+
+SET search_path = svartest;
+CREATE VARIABLE testvar as int;
+-- plpgsql variables are preferred against session variables
+DO $$
+<<myblock>>
+DECLARE testvar int;
+BEGIN
+  -- should be ok without warning
+  LET testvar = 100;
+  -- should be ok without warning
+  testvar := 1000;
+  -- should be ok without warning
+  RAISE NOTICE 'session variable is %', svartest.testvar;
+  -- should be ok without warning
+  RAISE NOTICE 'plpgsql variable is %', myblock.testvar;
+  -- should to print plpgsql variable with warning
+  RAISE NOTICE 'variable is %', testvar;
+END;
+$$;
+NOTICE:  session variable is 100
+NOTICE:  plpgsql variable is 1000
+WARNING:  session variable "testvar" is shadowed
+LINE 1: testvar
+        ^
+DETAIL:  Session variables can be shadowed by columns, routine's variables and routine's arguments with same name.
+QUERY:  testvar
+NOTICE:  variable is 1000
+DROP VARIABLE testvar;
+SET session_variables_ambiguity_warning TO default;
+-- should be ok
+SELECT avar FROM xxtab;
+ avar 
+------
+   10
+(1 row)
+
+CREATE VARIABLE public.avar AS int;
+-- should to fail
+SELECT avar FROM xxtab;
+ avar 
+------
+   10
+(1 row)
+
+-- should be ok
+SELECT public.avar FROM xxtab;
+ avar 
+------
+     
+(1 row)
+
+DROP VARIABLE xxtab;
+SELECT xxtab.avar FROM xxtab;
+ avar 
+------
+   10
+(1 row)
+
+DROP VARIABLE public.avar;
+DROP TYPE xxtype;
+DROP TABLE xxtab;
+-- test of plan cache invalidation
+CREATE VARIABLE xx AS int;
+SET plan_cache_mode = force_generic_plan;
+PREPARE pp AS SELECT xx;
+EXECUTE pp;
+ xx 
+----
+   
+(1 row)
+
+DROP VARIABLE xx;
+CREATE VARIABLE xx AS int;
+-- should to work
+EXECUTE pp;
+ xx 
+----
+   
+(1 row)
+
+DROP VARIABLE xx;
+DEALLOCATE pp;
+SET plan_cache_mode = DEFAULT;
diff --git a/src/test/regress/parallel_schedule b/src/test/regress/parallel_schedule
index 861c30a73a..410ef2850d 100644
--- a/src/test/regress/parallel_schedule
+++ b/src/test/regress/parallel_schedule
@@ -119,7 +119,7 @@ test: json jsonb json_encoding jsonpath jsonpath_encoding jsonb_jsonpath
 # NB: temp.sql does a reconnect which transiently uses 2 connections,
 # so keep this parallel group to at most 19 tests
 # ----------
-test: plancache limit plpgsql copy2 temp domain rangefuncs prepare conversion truncate alter_table sequence polymorphism rowtypes returning largeobject with xml
+test: plancache limit plpgsql copy2 temp domain rangefuncs prepare conversion truncate alter_table sequence polymorphism rowtypes returning largeobject with xml session_variables
 
 # ----------
 # Another group of parallel tests
diff --git a/src/test/regress/sql/session_variables.sql b/src/test/regress/sql/session_variables.sql
new file mode 100644
index 0000000000..3c184382c6
--- /dev/null
+++ b/src/test/regress/sql/session_variables.sql
@@ -0,0 +1,614 @@
+CREATE SCHEMA svartest;
+
+SET search_path = svartest;
+
+CREATE VARIABLE var1 AS integer;
+CREATE TEMP VARIABLE var2 AS text;
+
+DROP VARIABLE var1, var2;
+
+-- functional interface
+CREATE VARIABLE var1 AS numeric;
+
+CREATE ROLE var_test_role;
+GRANT USAGE ON SCHEMA svartest TO var_test_role;
+
+SET ROLE TO var_test_role;
+
+-- should fail
+SELECT var1;
+
+SET ROLE TO DEFAULT;
+
+GRANT READ ON VARIABLE var1 TO var_test_role;
+
+SET ROLE TO var_test_role;
+-- should fail
+LET var1 = 10;
+-- should work
+SELECT var1;
+
+SET ROLE TO DEFAULT;
+
+GRANT WRITE ON VARIABLE var1 TO var_test_role;
+
+SET ROLE TO var_test_role;
+
+-- should work
+LET var1 = 333;
+
+SET ROLE TO DEFAULT;
+
+REVOKE ALL ON VARIABLE var1 FROM var_test_role;
+
+CREATE OR REPLACE FUNCTION secure_var()
+RETURNS int AS $$
+  SELECT svartest.var1::int;
+$$ LANGUAGE sql SECURITY DEFINER;
+
+SELECT secure_var();
+
+SET ROLE TO var_test_role;
+
+-- should fail
+SELECT svartest.var1;
+
+-- should work;
+SELECT secure_var();
+
+SET ROLE TO DEFAULT;
+
+EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM generate_series(1,100) g(v) WHERE v = var1;
+
+CREATE VIEW schema_var_view AS SELECT var1;
+
+SELECT * FROM schema_var_view;
+
+\c -
+
+SET search_path = svartest;
+
+-- should work still, but var will be empty
+SELECT * FROM schema_var_view;
+
+LET var1 = pi();
+
+SELECT var1;
+
+-- we can see execution plan of LET statement
+EXPLAIN (VERBOSE, COSTS OFF) LET var1 = pi();
+
+SELECT var1;
+
+CREATE VARIABLE var3 AS int;
+
+CREATE OR REPLACE FUNCTION inc(int)
+RETURNS int AS $$
+BEGIN
+  LET svartest.var3 = COALESCE(svartest.var3 + $1, $1);
+  RETURN var3;
+END;
+$$ LANGUAGE plpgsql;
+
+SELECT inc(1);
+SELECT inc(1);
+SELECT inc(1);
+
+SELECT inc(1) FROM generate_series(1,10);
+
+SET ROLE TO var_test_role;
+
+-- should fail
+LET var3 = 0;
+
+SET ROLE TO DEFAULT;
+
+DROP VIEW schema_var_view;
+
+DROP VARIABLE var1 CASCADE;
+DROP VARIABLE var3 CASCADE;
+
+-- composite variables
+
+CREATE TYPE sv_xyz AS (x int, y int, z numeric(10,2));
+
+CREATE VARIABLE v1 AS sv_xyz;
+CREATE VARIABLE v2 AS sv_xyz;
+
+\d v1
+\d v2
+
+LET v1 = (1,2,3.14);
+LET v2 = (10,20,3.14*10);
+
+-- should work too - there are prepared casts
+LET v1 = (1,2,3.14);
+
+SELECT v1;
+SELECT v2;
+SELECT (v1).*;
+SELECT (v2).*;
+
+SELECT v1.x + v1.z;
+SELECT v2.x + v2.z;
+
+-- access to composite fields should be safe too
+-- should fail
+SET ROLE TO var_test_role;
+
+SELECT v2.x;
+
+SET ROLE TO DEFAULT;
+
+DROP VARIABLE v1;
+DROP VARIABLE v2;
+
+REVOKE USAGE ON SCHEMA svartest FROM var_test_role;
+DROP ROLE var_test_role;
+
+-- scalar variables should not be in conflict with qualified column
+CREATE VARIABLE varx AS text;
+SELECT varx.relname FROM pg_class varx WHERE varx.relname = 'pg_class';
+
+-- should fail
+SELECT varx.xxx;
+
+-- variables can be updated under RO transaction
+
+BEGIN;
+SET TRANSACTION READ ONLY;
+LET varx = 'hello';
+COMMIT;
+
+SELECT varx;
+
+DROP VARIABLE varx;
+
+CREATE TYPE t1 AS (a int, b numeric, c text);
+
+CREATE VARIABLE v1 AS t1;
+LET v1 = (1, pi(), 'hello');
+SELECT v1;
+LET v1.b = 10.2222;
+SELECT v1;
+
+-- should fail
+LET v1.x = 10;
+
+DROP VARIABLE v1;
+DROP TYPE t1;
+
+-- arrays are supported
+CREATE VARIABLE va1 AS numeric[];
+LET va1 = ARRAY[1.1,2.1];
+LET va1[1] = 10.1;
+SELECT va1;
+
+CREATE TYPE ta2 AS (a numeric, b numeric[]);
+CREATE VARIABLE va2 AS ta2;
+LET va2 = (10.1, ARRAY[0.0, 0.0]);
+LET va2.a = 10.2;
+SELECT va2;
+LET va2.b[1] = 10.3;
+SELECT va2;
+
+DROP VARIABLE va1;
+DROP VARIABLE va2;
+DROP TYPE ta2;
+
+-- default values
+CREATE VARIABLE v1 AS numeric DEFAULT pi();
+LET v1 = v1 * 2;
+SELECT v1;
+
+CREATE TYPE t2 AS (a numeric, b text);
+CREATE VARIABLE v2 AS t2 DEFAULT (NULL, 'Hello');
+LET svartest.v2.a = pi();
+SELECT v2;
+
+-- should fail due dependency
+DROP TYPE t2;
+
+-- should be ok
+DROP VARIABLE v1;
+DROP VARIABLE v2;
+
+-- tests of alters
+CREATE SCHEMA var_schema1;
+CREATE SCHEMA var_schema2;
+
+CREATE VARIABLE var_schema1.var1 AS integer;
+LET var_schema1.var1 = 1000;
+SELECT var_schema1.var1;
+ALTER VARIABLE var_schema1.var1 SET SCHEMA var_schema2;
+SELECT var_schema2.var1;
+
+CREATE ROLE var_test_role;
+
+ALTER VARIABLE var_schema2.var1 OWNER TO var_test_role;
+SET ROLE TO var_test_role;
+
+-- should fail, no access to schema var_schema2.var
+SELECT var_schema2.var1;
+DROP VARIABLE var_schema2.var1;
+
+SET ROLE TO DEFAULT;
+
+ALTER VARIABLE var_schema2.var1 SET SCHEMA public;
+
+SET ROLE TO var_test_role;
+SELECT public.var1;
+
+ALTER VARIABLE public.var1 RENAME TO var1_renamed;
+
+SELECT public.var1_renamed;
+
+DROP VARIABLE public.var1_renamed;
+
+SET ROLE TO DEFAULt;
+
+-- default rights test
+ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT ALL ON VARIABLES TO var_test_role;
+
+CREATE VARIABLE public.var2 AS int;
+
+SET ROLE TO var_test_role;
+
+-- should be ok
+LET public.var2 = 100;
+SELECT public.var2;
+
+SET ROLE TO DEFAULt;
+
+DROP VARIABLE public.var2;
+DROP OWNED BY var_test_role;
+
+DROP ROLE var_test_role;
+
+CREATE VARIABLE xx AS text DEFAULT 'hello';
+
+SELECT xx, upper(xx);
+
+LET xx = 'Hi';
+
+SELECT xx;
+
+DROP VARIABLE xx;
+
+-- ON TRANSACTION END RESET tests
+CREATE VARIABLE t1 AS int DEFAULT -1 ON TRANSACTION END RESET;
+
+BEGIN;
+  SELECT t1;
+  LET t1 = 100;
+  SELECT t1;
+COMMIT;
+
+SELECT t1;
+
+BEGIN;
+  SELECT t1;
+  LET t1 = 100;
+  SELECT t1;
+ROLLBACK;
+
+SELECT t1;
+
+DROP VARIABLE t1;
+
+CREATE VARIABLE v1 AS int DEFAULT 0;
+CREATE VARIABLE v2 AS text DEFAULT 'none';
+
+LET v1 = 100;
+LET v2 = 'Hello';
+SELECT v1, v2;
+LET v1 = DEFAULT;
+LET v2 = DEFAULT;
+SELECT v1, v2;
+
+DROP VARIABLE v1;
+DROP VARIABLE v2;
+
+-- ON COMMIT DROP tests
+-- should be 0 always
+SELECT count(*) FROM pg_variable;
+
+CREATE TEMP VARIABLE g AS int ON COMMIT DROP;
+
+SELECT count(*) FROM pg_variable;
+
+BEGIN;
+  CREATE TEMP VARIABLE g AS int ON COMMIT DROP;
+COMMIT;
+
+SELECT count(*) FROM pg_variable;
+
+BEGIN;
+  CREATE TEMP VARIABLE g AS int ON COMMIT DROP;
+ROLLBACK;
+
+SELECT count(*) FROM pg_variable;
+
+-- test on query with workers
+CREATE TABLE svar_test(a int);
+INSERT INTO svar_test SELECT * FROM generate_series(1,1000000);
+ANALYZE svar_test;
+CREATE VARIABLE zero int;
+LET zero = 0;
+
+-- parallel workers should be used
+EXPLAIN (costs off) SELECT count(*) FROM svar_test WHERE a%10 = zero;
+
+-- result should be 100000
+SELECT count(*) FROM svar_test WHERE a%10 = zero;
+
+LET zero = (SELECT count(*) FROM svar_test);
+
+-- result should be 1000000
+SELECT zero;
+
+-- parallel workers should be used
+EXPLAIN (costs off) LET zero = (SELECT count(*) FROM svar_test);
+
+DROP TABLE svar_test;
+DROP VARIABLE zero;
+
+-- use variables in prepared statements
+CREATE VARIABLE v AS numeric;
+LET v = 3.14;
+
+-- use variables in views
+CREATE VIEW vv AS SELECT COALESCE(v, 0) + 1000 AS result;
+SELECT * FROM vv;
+
+-- start a new session
+\c
+
+SET search_path to svartest;
+
+SELECT * FROM vv;
+LET v = 3.14;
+SELECT * FROM vv;
+
+-- should fail, dependency
+DROP VARIABLE v;
+
+-- should be ok
+DROP VARIABLE v CASCADE;
+
+-- other features
+CREATE VARIABLE dt AS integer DEFAULT 0;
+
+LET dt = 100;
+SELECT dt;
+
+DISCARD VARIABLES;
+
+SELECT dt;
+
+DROP VARIABLE dt;
+
+-- NOT NULL
+CREATE VARIABLE v1 AS int NOT NULL;
+CREATE VARIABLE v2 AS int NOT NULL DEFAULT NULL;
+
+-- should fail
+SELECT v1;
+SELECT v2;
+LET v1 = NULL;
+LET v2 = NULL;
+LET v1 = DEFAULT;
+LET v2 = DEFAULT;
+
+-- should be ok
+LET v1 = 100;
+LET v2 = 1000;
+SELECT v1, v2;
+
+DROP VARIABLE v1;
+DROP VARIABLE v2;
+
+CREATE VARIABLE tv AS int;
+CREATE VARIABLE IF NOT EXISTS tv AS int;
+DROP VARIABLE tv;
+
+CREATE IMMUTABLE VARIABLE iv AS int DEFAULT 100;
+SELECT iv;
+
+-- should fail;
+LET iv = 10000;
+
+DROP VARIABLE iv;
+
+-- different order
+CREATE IMMUTABLE VARIABLE iv AS int DEFAULT 100;
+-- should to fail
+LET iv = 10000;
+-- should be ok
+SELECT iv;
+
+DROP VARIABLE iv;
+
+CREATE IMMUTABLE VARIABLE iv AS int;
+
+-- should be ok
+LET iv = NULL;
+LET iv = NULL;
+
+-- should be ok
+LET iv = 10;
+
+-- should fail
+LET iv = 10;
+
+DROP VARIABLE iv;
+
+-- create variable inside plpgsql block
+DO $$
+BEGIN
+  CREATE VARIABLE do_test_svar AS date DEFAULT '2000-01-01';
+END;
+$$;
+
+SELECT do_test_svar;
+
+DROP VARIABLE do_test_svar;
+
+-- should fail
+CREATE IMMUTABLE VARIABLE xx AS int NOT NULL;
+
+
+
+-- REASSIGN OWNED test
+CREATE ROLE var_test_role1;
+CREATE ROLE var_test_role2;
+
+CREATE VARIABLE xxx_var AS int;
+
+ALTER VARIABLE xxx_var OWNER TO var_test_role1;
+REASSIGN OWNED BY var_test_role1 to var_test_role2;
+
+SELECT varowner::regrole FROM pg_variable WHERE varname = 'xxx_var';
+
+DROP OWNED BY var_test_role1;
+DROP ROLE var_test_role1;
+SELECT count(*) FROM pg_variable WHERE varname = 'xxx_var';
+
+DROP OWNED BY var_test_role2;
+DROP ROLE var_test_role2;
+SELECT count(*) FROM pg_variable WHERE varname = 'xxx_var';
+
+-- creating, dropping temporal variable
+BEGIN;
+
+CREATE TEMP VARIABLE tempvar AS INT ON COMMIT DROP;
+
+LET tempvar = 100;
+
+SAVEPOINT s1;
+
+DROP VARIABLE tempvar;
+
+ROLLBACK TO s1;
+
+SELECT tempvar;
+
+COMMIT;
+
+-- should to fail
+LET tempvar = 100;
+
+BEGIN;
+
+SAVEPOINT s1;
+
+CREATE TEMP VARIABLE tempvar AS INT ON COMMIT DROP;
+
+LET tempvar = 100;
+
+ROLLBACK TO s1;
+
+COMMIT;
+
+-- should to fail
+LET tempvar = 100;
+
+CREATE VARIABLE var1 AS int;
+LET var1 = 100;
+BEGIN;
+DROP VARIABLE var1;
+ROLLBACK;
+SELECT var1;
+
+DROP VARIABLE var1;
+
+CREATE VARIABLE var1 AS int DEFAULT 100;
+COMMENT ON VARIABLE var1 IS 'some variable comment';
+
+SELECT pg_catalog.obj_description(oid, 'pg_variable') FROM pg_variable WHERE varname = 'var1';
+
+DROP VARIABLE var1;
+
+CREATE TABLE xxtab(avar int);
+
+CREATE TYPE xxtype AS (avar int);
+
+CREATE VARIABLE xxtab AS xxtype;
+
+INSERT INTO xxtab VALUES(10);
+
+-- it is ambiguous, but columns are preferred
+SELECT xxtab.avar FROM xxtab;
+
+SET session_variables_ambiguity_warning TO on;
+
+SELECT xxtab.avar FROM xxtab;
+
+SET search_path = svartest;
+
+CREATE VARIABLE testvar as int;
+
+-- plpgsql variables are preferred against session variables
+DO $$
+<<myblock>>
+DECLARE testvar int;
+BEGIN
+  -- should be ok without warning
+  LET testvar = 100;
+  -- should be ok without warning
+  testvar := 1000;
+  -- should be ok without warning
+  RAISE NOTICE 'session variable is %', svartest.testvar;
+  -- should be ok without warning
+  RAISE NOTICE 'plpgsql variable is %', myblock.testvar;
+  -- should to print plpgsql variable with warning
+  RAISE NOTICE 'variable is %', testvar;
+END;
+$$;
+
+DROP VARIABLE testvar;
+
+SET session_variables_ambiguity_warning TO default;
+
+-- should be ok
+SELECT avar FROM xxtab;
+
+CREATE VARIABLE public.avar AS int;
+
+-- should to fail
+SELECT avar FROM xxtab;
+
+-- should be ok
+SELECT public.avar FROM xxtab;
+
+DROP VARIABLE xxtab;
+
+SELECT xxtab.avar FROM xxtab;
+
+DROP VARIABLE public.avar;
+
+DROP TYPE xxtype;
+
+DROP TABLE xxtab;
+
+-- test of plan cache invalidation
+CREATE VARIABLE xx AS int;
+
+SET plan_cache_mode = force_generic_plan;
+
+PREPARE pp AS SELECT xx;
+
+EXECUTE pp;
+
+DROP VARIABLE xx;
+
+CREATE VARIABLE xx AS int;
+
+-- should to work
+EXECUTE pp;
+
+DROP VARIABLE xx;
+
+DEALLOCATE pp;
+
+SET plan_cache_mode = DEFAULT;
-- 
2.33.1

