Last year, I questioned why CREATE TRIGGER acquires an
AccessExclusiveLock on its target table:
http://archives.postgresql.org/pgsql-hackers/2005-03/msg00764.php
Acquiring an ExclusiveLock should be sufficient: we can safely allow
concurrent SELECTs on the table. (The -hackers thread discusses both
CREATE TRIGGER and ALTER TABLE ADD FK; the latter might require some
more consideration, so I'll tackle that later.)
This patch implements this change, and updates the documentation.
Barring any objections, I'll apply this in a day or two.
-Neil
Index: doc/src/sgml/mvcc.sgml
===================================================================
RCS file: /projects/cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v
retrieving revision 2.56
diff -c -r2.56 mvcc.sgml
*** doc/src/sgml/mvcc.sgml 23 Apr 2006 03:39:52 -0000 2.56
--- doc/src/sgml/mvcc.sgml 4 Jul 2006 06:21:07 -0000
***************
*** 683,691 ****
</para>
<para>
! This lock mode is not automatically acquired on user tables by any
! <productname>PostgreSQL</productname> command. However it is
! acquired on certain system catalogs in some operations.
</para>
</listitem>
</varlistentry>
--- 683,691 ----
</para>
<para>
! Acquired by <command>CREATE TRIGGER</>. This lock mode is also
! acquired by <productname>PostgreSQL</> on certain system catalogs in
! some operations.
</para>
</listitem>
</varlistentry>
Index: src/backend/commands/trigger.c
===================================================================
RCS file: /projects/cvsroot/pgsql/src/backend/commands/trigger.c,v
retrieving revision 1.203
diff -c -r1.203 trigger.c
*** src/backend/commands/trigger.c 16 Jun 2006 20:23:44 -0000 1.203
--- src/backend/commands/trigger.c 4 Jul 2006 06:21:13 -0000
***************
*** 92,98 ****
ObjectAddress myself,
referenced;
! rel = heap_openrv(stmt->relation, AccessExclusiveLock);
if (stmt->constrrel != NULL)
constrrelid = RangeVarGetRelid(stmt->constrrel, false);
--- 92,102 ----
ObjectAddress myself,
referenced;
! /*
! * Make sure we block concurrent data-modifying commands and DDL, but we
! * can safely allow SELECTs.
! */
! rel = heap_openrv(stmt->relation, ExclusiveLock);
if (stmt->constrrel != NULL)
constrrelid = RangeVarGetRelid(stmt->constrrel, false);
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