diff --git a/doc/src/sgml/mvcc.sgml b/doc/src/sgml/mvcc.sgml
index 1aeec22b66cf43a24755f861d0827a42bfc03838..5982c22bd3339f74227c5edd6d841cfa2e3d01ca 100644
--- a/doc/src/sgml/mvcc.sgml
+++ b/doc/src/sgml/mvcc.sgml
@@ -1,5 +1,5 @@
 <!--
-$PostgreSQL: pgsql/doc/src/sgml/mvcc.sgml,v 2.47 2005/02/26 18:37:17 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/mvcc.sgml,v 2.48 2005/03/23 07:44:56 neilc Exp $
 -->
 
  <chapter id="mvcc">
@@ -677,9 +677,10 @@ SELECT SUM(value) FROM mytab WHERE class = 2;
 	</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.
+         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.
 	</para>
        </listitem>
       </varlistentry>
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 0b11976db848aa9ffd067847c8e5439f8245ca92..1a6c1f82625e0230ab56d01f841ada6472aa8c48 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.148 2005/03/20 22:00:52 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.149 2005/03/23 07:44:57 neilc Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -3829,13 +3829,13 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 	Oid			constrOid;
 
 	/*
-	 * 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.)
+	 * 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.
 	 */
-	pkrel = heap_openrv(fkconstraint->pktable, AccessExclusiveLock);
+	pkrel = heap_openrv(fkconstraint->pktable, ExclusiveLock);
 
 	/*
 	 * Validity and permissions checks
diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index fd7d9afb836f59ea94d30b32b65cb3c3e9a38d37..55333e1f4f108f5414fba3d3d1885aa52fccdafa 100644
--- a/src/backend/commands/trigger.c
+++ b/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 1.178 2005/03/20 23:40:24 neilc Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/trigger.c,v 1.179 2005/03/23 07:44:57 neilc Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -87,7 +87,14 @@ 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);