Revert changes to CREATE TRIGGER and ALTER TABLE ADD FOREIGN KEY locking,

per request from Tom.

doc/src/sgml/mvcc.sgml

@@ -1,5 +1,5 @@
 <!--
-$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">
@@ -677,10 +677,9 @@ SELECT SUM(value) FROM mytab WHERE class = 2;
 	</para>
 
 	<para>
-         Acquired by <command>CREATE TRIGGER</command> and
+         This lock mode is not automatically acquired on user tables by any
-         <command>ALTER TABLE ADD FOREIGN KEY</command>. This lock
+         <productname>PostgreSQL</productname> command.  However it is
-         mode can also be acquired on certain system catalogs in some
+         acquired on certain system catalogs in some operations.
-         operations.
 	</para>
        </listitem>
       </varlistentry>

src/backend/commands/tablecmds.c

@@ -8,7 +8,7 @@
  *
  *
  * 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 $
  *
  *-------------------------------------------------------------------------
  */
@@ -3829,13 +3829,13 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 	Oid			constrOid;
 
 	/*
-	 * Grab a lock on the pk table, so that someone doesn't delete
+	 * Grab an exclusive lock on the pk table, so that someone doesn't
-	 * rows out from under us. We will eventually need to add triggers
+	 * delete rows out from under us. (Although a lesser lock would do for
-	 * to the table, at which point we'll need to an ExclusiveLock --
+	 * that purpose, we'll need exclusive lock anyway to add triggers to
-	 * therefore we grab an ExclusiveLock now to prevent possible
+	 * the pk table; trying to start with a lesser lock will just create a
-	 * deadlock.
+	 * risk of deadlock.)
 	 */
-	pkrel = heap_openrv(fkconstraint->pktable, ExclusiveLock);
+	pkrel = heap_openrv(fkconstraint->pktable, AccessExclusiveLock);
 
 	/*
 	 * Validity and permissions checks

src/backend/commands/trigger.c

@@ -7,7 +7,7 @@
  * 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 $
  *
  *-------------------------------------------------------------------------
  */
@@ -87,14 +87,7 @@ CreateTrigger(CreateTrigStmt *stmt, bool forConstraint)
 	ObjectAddress myself,
 				referenced;
 
-    /*
+	rel = heap_openrv(stmt->relation, AccessExclusiveLock);
-     * 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);
 
 	if (stmt->constrrel != NULL)
 		constrrelid = RangeVarGetRelid(stmt->constrrel, false);