Adjust CREATE TRIGGER and ALTER TABLE ... ADD FOREIGN KEY to acquire

ExclusiveLock rather than AccessExclusiveLock. This will allow concurrent
SELECT queries to proceed on the table. Per discussion with Andrew at
SuperNews.

diff --git a/doc/src/sgml/mvcc.sgml b/doc/src/sgml/mvcc.sgml
index 1aeec22b66cf43a24755f861d0827a42bfc03838..5982c22bd3339f74227c5edd6d841cfa2e3d01ca 100644 (file)
--- 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 (file)
--- 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 (file)
--- 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);