diff --git a/doc/src/sgml/mvcc.sgml b/doc/src/sgml/mvcc.sgml
index 6352e12..9915265 100644
--- a/doc/src/sgml/mvcc.sgml
+++ b/doc/src/sgml/mvcc.sgml
@@ -698,6 +698,28 @@ ERROR: could not serialize access due to read/write dependencies among transact
for some environments.
+
+
+ PostgreSQL's Serializable transaction isolation level
+ only allows concurrent transactions to commit if it can prove there is a
+ serial order of execution that would produce the same effect. However,
+ it doesn't always prevent errors from being raised that would not occur
+ in true serial execution. In particular, it is possible to see unique
+ constraint violations caused by conflicts with overlapping Serializable
+ transactions even after explicitly checking that the key isn't present
+ before attempting to insert it. This can be avoided by making sure
+ that all Serializable transactions that insert potentially
+ conflicting keys explicitly check if they can do so first. For example,
+ imagine an application that asks the user for a new key and then checks
+ that it doesn't exist already by trying to select it first, or generates
+ a new key by selecting the maximum existing key and adding one. If some
+ Serializable transactions insert new keys directly without following this
+ protocol, unique constraints violations might be reported even in cases
+ where they could not occur in a serial execution of the concurrent
+ transactions.
+
+
+
For optimal performance when relying on Serializable transactions for
concurrency control, these issues should be considered:
diff --git a/src/backend/access/nbtree/nbtinsert.c b/src/backend/access/nbtree/nbtinsert.c
index e3c55eb..3e100aa 100644
--- a/src/backend/access/nbtree/nbtinsert.c
+++ b/src/backend/access/nbtree/nbtinsert.c
@@ -392,6 +392,14 @@ _bt_check_unique(Relation rel, IndexTuple itup, Relation heapRel,
}
/*
+ * Check for a conflict-in as we would if we were going to
+ * write to this page. We aren't actually going to write,
+ * but we want a chance to report SSI conflicts that would
+ * otherwise be masked by this unique constraint violation.
+ */
+ CheckForSerializableConflictIn(rel, NULL, buf);
+
+ /*
* This is a definite conflict. Break the tuple down into
* datums and report the error. But first, make sure we
* release the buffer locks we're holding ---
diff --git a/src/test/isolation/expected/read-write-unique-2.out b/src/test/isolation/expected/read-write-unique-2.out
new file mode 100644
index 0000000..5e27f0a
--- /dev/null
+++ b/src/test/isolation/expected/read-write-unique-2.out
@@ -0,0 +1,29 @@
+Parsed test spec with 2 sessions
+
+starting permutation: r1 r2 w1 w2 c1 c2
+step r1: SELECT * FROM test WHERE i = 42;
+i
+
+step r2: SELECT * FROM test WHERE i = 42;
+i
+
+step w1: INSERT INTO test VALUES (42);
+step w2: INSERT INTO test VALUES (42);
+step c1: COMMIT;
+step w2: <... completed>
+error in steps c1 w2: ERROR: could not serialize access due to read/write dependencies among transactions
+step c2: COMMIT;
+
+starting permutation: r1 w1 c1 r2 w2 c2
+step r1: SELECT * FROM test WHERE i = 42;
+i
+
+step w1: INSERT INTO test VALUES (42);
+step c1: COMMIT;
+step r2: SELECT * FROM test WHERE i = 42;
+i
+
+42
+step w2: INSERT INTO test VALUES (42);
+ERROR: duplicate key value violates unique constraint "test_pkey"
+step c2: COMMIT;
diff --git a/src/test/isolation/expected/read-write-unique-3.out b/src/test/isolation/expected/read-write-unique-3.out
new file mode 100644
index 0000000..edd3558
--- /dev/null
+++ b/src/test/isolation/expected/read-write-unique-3.out
@@ -0,0 +1,12 @@
+Parsed test spec with 2 sessions
+
+starting permutation: rw1 rw2 c1 c2
+step rw1: SELECT insert_unique(1, '1');
+insert_unique
+
+
+step rw2: SELECT insert_unique(1, '2');
+step c1: COMMIT;
+step rw2: <... completed>
+error in steps c1 rw2: ERROR: could not serialize access due to read/write dependencies among transactions
+step c2: COMMIT;
diff --git a/src/test/isolation/expected/read-write-unique-4.out b/src/test/isolation/expected/read-write-unique-4.out
new file mode 100644
index 0000000..64ff157
--- /dev/null
+++ b/src/test/isolation/expected/read-write-unique-4.out
@@ -0,0 +1,41 @@
+Parsed test spec with 2 sessions
+
+starting permutation: r1 r2 w1 w2 c1 c2
+step r1: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
+coalesce
+
+3
+step r2: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
+coalesce
+
+3
+step w1: INSERT INTO invoice VALUES (2016, 3);
+step w2: INSERT INTO invoice VALUES (2016, 3);
+step c1: COMMIT;
+step w2: <... completed>
+error in steps c1 w2: ERROR: could not serialize access due to read/write dependencies among transactions
+step c2: COMMIT;
+
+starting permutation: r1 w1 w2 c1 c2
+step r1: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
+coalesce
+
+3
+step w1: INSERT INTO invoice VALUES (2016, 3);
+step w2: INSERT INTO invoice VALUES (2016, 3);
+step c1: COMMIT;
+step w2: <... completed>
+error in steps c1 w2: ERROR: duplicate key value violates unique constraint "invoice_pkey"
+step c2: COMMIT;
+
+starting permutation: r2 w1 w2 c1 c2
+step r2: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
+coalesce
+
+3
+step w1: INSERT INTO invoice VALUES (2016, 3);
+step w2: INSERT INTO invoice VALUES (2016, 3);
+step c1: COMMIT;
+step w2: <... completed>
+error in steps c1 w2: ERROR: duplicate key value violates unique constraint "invoice_pkey"
+step c2: COMMIT;
diff --git a/src/test/isolation/expected/read-write-unique.out b/src/test/isolation/expected/read-write-unique.out
new file mode 100644
index 0000000..fb32ec3
--- /dev/null
+++ b/src/test/isolation/expected/read-write-unique.out
@@ -0,0 +1,29 @@
+Parsed test spec with 2 sessions
+
+starting permutation: r1 r2 w1 w2 c1 c2
+step r1: SELECT * FROM test;
+i
+
+step r2: SELECT * FROM test;
+i
+
+step w1: INSERT INTO test VALUES (42);
+step w2: INSERT INTO test VALUES (42);
+step c1: COMMIT;
+step w2: <... completed>
+error in steps c1 w2: ERROR: could not serialize access due to read/write dependencies among transactions
+step c2: COMMIT;
+
+starting permutation: r1 w1 c1 r2 w2 c2
+step r1: SELECT * FROM test;
+i
+
+step w1: INSERT INTO test VALUES (42);
+step c1: COMMIT;
+step r2: SELECT * FROM test;
+i
+
+42
+step w2: INSERT INTO test VALUES (42);
+ERROR: duplicate key value violates unique constraint "test_pkey"
+step c2: COMMIT;
diff --git a/src/test/isolation/isolation_schedule b/src/test/isolation/isolation_schedule
index 138a0b7..fbd2192 100644
--- a/src/test/isolation/isolation_schedule
+++ b/src/test/isolation/isolation_schedule
@@ -1,3 +1,7 @@
+test: read-write-unique
+test: read-write-unique-2
+test: read-write-unique-3
+test: read-write-unique-4
test: simple-write-skew
test: receipt-report
test: temporal-range-integrity
diff --git a/src/test/isolation/specs/read-write-unique-2.spec b/src/test/isolation/specs/read-write-unique-2.spec
new file mode 100644
index 0000000..5e7cbf2
--- /dev/null
+++ b/src/test/isolation/specs/read-write-unique-2.spec
@@ -0,0 +1,36 @@
+# Read-write-unique test.
+
+setup
+{
+ CREATE TABLE test (i integer PRIMARY KEY);
+}
+
+teardown
+{
+ DROP TABLE test;
+}
+
+session "s1"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r1" { SELECT * FROM test WHERE i = 42; }
+step "w1" { INSERT INTO test VALUES (42); }
+step "c1" { COMMIT; }
+
+session "s2"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r2" { SELECT * FROM test WHERE i = 42; }
+step "w2" { INSERT INTO test VALUES (42); }
+step "c2" { COMMIT; }
+
+# Two SSI transactions see that there is no row with value 42
+# in the table, then try to insert that value; T1 inserts,
+# and then T2 blocks waiting for T1 to commit. Finally,
+# T2 reports a serialization failure.
+
+permutation "r1" "r2" "w1" "w2" "c1" "c2"
+
+# If the value is already visible before T2 begins, then a
+# regular unique constraint violation should still be raised
+# by T2.
+
+permutation "r1" "w1" "c1" "r2" "w2" "c2"
diff --git a/src/test/isolation/specs/read-write-unique-3.spec b/src/test/isolation/specs/read-write-unique-3.spec
new file mode 100644
index 0000000..52d2877
--- /dev/null
+++ b/src/test/isolation/specs/read-write-unique-3.spec
@@ -0,0 +1,33 @@
+# Read-write-unique test.
+# From bug report 9301.
+
+setup
+{
+ CREATE TABLE test (
+ key integer UNIQUE,
+ val text
+ );
+
+ CREATE OR REPLACE FUNCTION insert_unique(k integer, v text) RETURNS void
+ LANGUAGE SQL AS $$
+ INSERT INTO test (key, val) SELECT k, v WHERE NOT EXISTS (SELECT key FROM test WHERE key = k);
+ $$;
+}
+
+teardown
+{
+ DROP FUNCTION insert_unique(integer, text);
+ DROP TABLE test;
+}
+
+session "s1"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "rw1" { SELECT insert_unique(1, '1'); }
+step "c1" { COMMIT; }
+
+session "s2"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "rw2" { SELECT insert_unique(1, '2'); }
+step "c2" { COMMIT; }
+
+permutation "rw1" "rw2" "c1" "c2"
diff --git a/src/test/isolation/specs/read-write-unique-4.spec b/src/test/isolation/specs/read-write-unique-4.spec
new file mode 100644
index 0000000..ec44782
--- /dev/null
+++ b/src/test/isolation/specs/read-write-unique-4.spec
@@ -0,0 +1,48 @@
+# Read-write-unique test.
+# Implementing a gapless sequence of ID numbers for each year.
+
+setup
+{
+ CREATE TABLE invoice (
+ year int,
+ invoice_number int,
+ PRIMARY KEY (year, invoice_number)
+ );
+
+ INSERT INTO invoice VALUES (2016, 1), (2016, 2);
+}
+
+teardown
+{
+ DROP TABLE invoice;
+}
+
+session "s1"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r1" { SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016; }
+step "w1" { INSERT INTO invoice VALUES (2016, 3); }
+step "c1" { COMMIT; }
+
+session "s2"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r2" { SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016; }
+step "w2" { INSERT INTO invoice VALUES (2016, 3); }
+step "c2" { COMMIT; }
+
+# if they both read first then there should be an SSI conflict
+permutation "r1" "r2" "w1" "w2" "c1" "c2"
+
+# cases where one session doesn't explicitly read before writing:
+
+# if s2 doesn't explicitly read, then trying to insert the value
+# generates a unique constraint violation after s1 commits, as if s2
+# ran after s1
+permutation "r1" "w1" "w2" "c1" "c2"
+
+# if s1 doesn't explicitly read, but s2 does, then s1 inserts and
+# commits first, should s2 experience an SSI failure instead of a
+# unique constraint violation? there is no serial order of operations
+# (s1, s2) or (s2, s1) where s1 succeeds, and s2 doesn't see the row
+# in an explicit select but then fails to insert due to unique
+# constraint violation
+permutation "r2" "w1" "w2" "c1" "c2"
diff --git a/src/test/isolation/specs/read-write-unique.spec b/src/test/isolation/specs/read-write-unique.spec
new file mode 100644
index 0000000..c782f10
--- /dev/null
+++ b/src/test/isolation/specs/read-write-unique.spec
@@ -0,0 +1,39 @@
+# Read-write-unique test.
+
+setup
+{
+ CREATE TABLE test (i integer PRIMARY KEY);
+}
+
+teardown
+{
+ DROP TABLE test;
+}
+
+session "s1"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r1" { SELECT * FROM test; }
+step "w1" { INSERT INTO test VALUES (42); }
+step "c1" { COMMIT; }
+
+session "s2"
+setup { BEGIN ISOLATION LEVEL SERIALIZABLE; }
+step "r2" { SELECT * FROM test; }
+step "w2" { INSERT INTO test VALUES (42); }
+step "c2" { COMMIT; }
+
+# Two SSI transactions see that there is no row with value 42
+# in the table, then try to insert that value; T1 inserts,
+# and then T2 blocks waiting for T1 to commit. Finally,
+# T2 reports a serialization failure.
+#
+# (In an earlier version of Postgres, T2 would report a unique
+# constraint violation).
+
+permutation "r1" "r2" "w1" "w2" "c1" "c2"
+
+# If the value is already visible before T2 begins, then a
+# regular unique constraint violation should still be raised
+# by T2.
+
+permutation "r1" "w1" "c1" "r2" "w2" "c2"