Reduce lock levels of CREATE TRIGGER and some ALTER TABLE, CREATE RULE actions.

Avoid hard-coding lockmode used for many altering DDL commands, allowing easier
future changes of lock levels. Implementation of initial analysis on DDL
sub-commands, so that many lock levels are now at ShareUpdateExclusiveLock or
ShareRowExclusiveLock, allowing certain DDL not to block reads/writes.
First of number of planned changes in this area; additional docs required
when full project complete.

Author: simonat2ndQuadrant

doc/src/sgml/mvcc.sgml

@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/mvcc.sgml,v 2.75 2010/05/03 15:35:30 alvherre Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/mvcc.sgml,v 2.76 2010/07/28 05:22:24 sriggs Exp $ -->
 
  <chapter id="mvcc">
   <title>Concurrency Control</title>
@@ -532,7 +532,7 @@ SELECT SUM(value) FROM mytab WHERE class = 2;
     most <productname>PostgreSQL</productname> commands automatically
     acquire locks of appropriate modes to ensure that referenced
     tables are not dropped or modified in incompatible ways while the
-    command executes.  (For example, <command>ALTER TABLE</> cannot safely be
+    command executes.  (For example, <command>TRUNCATE</> cannot safely be
     executed concurrently with other operations on the same table, so it
     obtains an exclusive lock on the table to enforce that.)
    </para>
@@ -695,8 +695,9 @@ SELECT SUM(value) FROM mytab WHERE class = 2;
         </para>
 
         <para>
-         This lock mode is not automatically acquired by any
-         <productname>PostgreSQL</productname> command.
+         Acquired by <command>CREATE TRIGGER</command>,
+         <command>CREATE RULE</command> (except for <literal>ON SELECT</>
+         rules) and in some cases <command>ALTER TABLE</command>.
         </para>
        </listitem>
       </varlistentry>
@@ -742,11 +743,12 @@ SELECT SUM(value) FROM mytab WHERE class = 2;
         </para>
 
         <para>
-         Acquired by the <command>ALTER TABLE</command>, <command>DROP
-         TABLE</command>, <command>TRUNCATE</command>, <command>REINDEX</command>,
+         Acquired by the <command>DROP TABLE</command>,
+         <command>TRUNCATE</command>, <command>REINDEX</command>,
          <command>CLUSTER</command>, and <command>VACUUM FULL</command>
-         commands.  This is also the default lock mode for <command>LOCK
-         TABLE</command> statements that do not specify a mode explicitly.
+         commands, as well as most variants of <command>ALTER TABLE</>.
+         This is also the default lock mode for <command>LOCK TABLE</command>
+         statements that do not specify a mode explicitly.
         </para>
        </listitem>
       </varlistentry>

src/backend/catalog/pg_shdepend.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/catalog/pg_shdepend.c,v 1.43 2010/07/06 19:18:56 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/catalog/pg_shdepend.c,v 1.44 2010/07/28 05:22:24 sriggs Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1346,7 +1346,7 @@ shdepReassignOwned(List *roleids, Oid newrole)
 					 * owned sequences, etc when we happen to visit them
 					 * before their parent table.
 					 */
-					ATExecChangeOwner(sdepForm->objid, newrole, true);
+					ATExecChangeOwner(sdepForm->objid, newrole, true, AccessExclusiveLock);
 					break;
 
 				case ProcedureRelationId:

src/backend/commands/alter.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/alter.c,v 1.36 2010/06/13 17:43:12 rhaas Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/alter.c,v 1.37 2010/07/28 05:22:24 sriggs Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -190,7 +190,7 @@ ExecAlterObjectSchemaStmt(AlterObjectSchemaStmt *stmt)
 		case OBJECT_VIEW:
 			CheckRelationOwnership(stmt->relation, true);
 			AlterTableNamespace(stmt->relation, stmt->newschema,
-								stmt->objectType);
+								stmt->objectType, AccessExclusiveLock);
 			break;
 
 		case OBJECT_TYPE: