Re: [WIP] The relminxid addition, try 3 - Mailing list pgsql-patches

Alvaro Herrera wrote:

Hi,

> Here is a reworked relminxid patch.  I added XLOG operations for heap
> unfreeze and database unfreeze.  The latter happens when someone
> connects to a frozen database.  The former happens when a LockRelation()
> is called on a frozen relation, and the lock is stronger than
> AccessShare.

I've been further reworking this patch.  It is basically as discussed:
tables are unfrozen when LockRelation() is called, databases are
unfrozen when they are connected to.  Those operations are performed
using heap_inplace_update, so we no longer need to take particular steps
to XLog them.

There are two new columns in pg_class (relminxid and relvacuumxid).
Both of them can be set to FrozenTransactionId.  The minimum of each is
used to determine database-wide datminxid and datvacuumxid.  Those can
also be to FrozenXid.  (Note that it seems to be problematic that all
databases have FrozenXids; however, this can only happen in a standalone
backend, when you VACUUM FREEZE a database and all other databases are
already frozen.  So we can truncate to CurrentTransactionId without
problem.)

However there's a big problem once again but I don't know how to fix it.
I don't know why this didn't show up before.  Consider the following
scenario:

- pg_attribute is frozen
- pg_class is frozen

CREATE TABLE foo (a int);

for some unknown reason, an inval message involving relation foo seems
to be emitted.

heap_unfreeze(pg_class)
  CommandCounterIncrement()
heap_unfreeze(pg_attribute)
  CommandCounterIncrement()
... insert the pg_attribute rows ...

During one of those CommandCounterIncrement calls, the "foo" relation
receives an invalidation message and the system tries to rebuild the
RelationDesc.  But at this point the pg_attribute rows are not there
yet, so the rebuild fails because it can't find them:

$ runpg 10relminxid client
Bienvenido a psql 8.2devel, la terminal interactiva de PostgreSQL.

Digite:  \copyright para ver los términos de distribución
       \h para ayuda de comandos SQL
       \? para ayuda de comandos psql
       \g o or termine con punto y coma para ejecutar una consulta
       \q para salir

alvherre=# vacuum freeze ;
alvherre=# \q

$ runpg 10relminxid client
Bienvenido a psql 8.2devel, la terminal interactiva de PostgreSQL.

Digite:  \copyright para ver los términos de distribución
       \h para ayuda de comandos SQL
       \? para ayuda de comandos psql
       \g o or termine con punto y coma para ejecutar una consulta
       \q para salir

alvherre=# create table hmmmm (a int);
NOTICE:  unfreezing pg_type
NOTICE:  unfreezing pg_depend
NOTICE:  unfreezing pg_shdepend
NOTICE:  unfreezing pg_attribute
ERROR:  catalog is missing 1 attribute(s) for relid 16409


Strangely this doesn't happen if I create the table right away at the
same connection that did the VACUUM FREEZE.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.