per request from Tom.
<!--
-$PostgreSQL: pgsql/doc/src/sgml/mvcc.sgml,v 2.48 2005/03/23 07:44:56 neilc Exp $
+$PostgreSQL: pgsql/doc/src/sgml/mvcc.sgml,v 2.49 2005/03/24 00:03:18 neilc Exp $
-->
<chapter id="mvcc">
</para>
<para>
- Acquired by <command>CREATE TRIGGER</command> and
- <command>ALTER TABLE ADD FOREIGN KEY</command>. This lock
- mode can also be acquired on certain system catalogs in some
- operations.
+ 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>
*
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.149 2005/03/23 07:44:57 neilc Exp $
+ * $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.150 2005/03/24 00:03:22 neilc Exp $
*
*-------------------------------------------------------------------------
*/
Oid constrOid;
/*
- * Grab a lock on the pk table, so that someone doesn't delete
- * rows out from under us. We will eventually need to add triggers
- * to the table, at which point we'll need to an ExclusiveLock --
- * therefore we grab an ExclusiveLock now to prevent possible
- * deadlock.
+ * Grab an exclusive lock on the pk table, so that someone doesn't
+ * delete rows out from under us. (Although a lesser lock would do for
+ * that purpose, we'll need exclusive lock anyway to add triggers to
+ * the pk table; trying to start with a lesser lock will just create a
+ * risk of deadlock.)
*/
- pkrel = heap_openrv(fkconstraint->pktable, ExclusiveLock);
+ pkrel = heap_openrv(fkconstraint->pktable, AccessExclusiveLock);
/*
* Validity and permissions checks
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/commands/trigger.c,v 1.179 2005/03/23 07:44:57 neilc Exp $
+ * $PostgreSQL: pgsql/src/backend/commands/trigger.c,v 1.180 2005/03/24 00:03:26 neilc Exp $
*
*-------------------------------------------------------------------------
*/
ObjectAddress myself,
referenced;
- /*
- * We need to prevent concurrent CREATE TRIGGER commands, as well
- * as concurrent table modifications (INSERT, DELETE, UPDATE), so
- * acquire an ExclusiveLock -- it should be fine to allow SELECTs
- * to proceed. We could perhaps acquire ShareRowExclusiveLock, but
- * there seems little gain in allowing SELECT FOR UPDATE.
- */
- rel = heap_openrv(stmt->relation, ExclusiveLock);
+ rel = heap_openrv(stmt->relation, AccessExclusiveLock);
if (stmt->constrrel != NULL)
constrrelid = RangeVarGetRelid(stmt->constrrel, false);