Finish disabling reduced-lock-levels-for-DDL feature.

Previous patch only covered the ALTER TABLE changes, not changes in other
commands; and it neglected to revert the documentation changes.

doc/src/sgml/mvcc.sgml

@@ -878,9 +878,8 @@ ERROR:  could not serialize access due to read/write dependencies among transact
         </para>
 
         <para>
-         Acquired by <command>CREATE TRIGGER</command>,
-         <command>CREATE RULE</command> (except for <literal>ON SELECT</>
-         rules) and some forms of <command>ALTER TABLE</command>.
+         This lock mode is not automatically acquired by any
+         <productname>PostgreSQL</productname> command.
         </para>
        </listitem>
       </varlistentry>
@@ -925,10 +924,10 @@ ERROR:  could not serialize access due to read/write dependencies among transact
         </para>
 
         <para>
-         Acquired by the <command>DROP TABLE</command>,
+         Acquired by the <command>ALTER TABLE</>, <command>DROP TABLE</>,
          <command>TRUNCATE</command>, <command>REINDEX</command>,
          <command>CLUSTER</command>, and <command>VACUUM FULL</command>
-         commands, and some forms of <command>ALTER TABLE</>.
+         commands.
          This is also the default lock mode for <command>LOCK TABLE</command>
          statements that do not specify a mode explicitly.
         </para>

src/backend/commands/tablecmds.c

@@ -3425,14 +3425,12 @@ ATRewriteTables(List **wqueue, LOCKMODE lockmode)
 				Relation	refrel;
 
 				if (rel == NULL)
+				{
 					/* Long since locked, no need for another */
 					rel = heap_open(tab->relid, NoLock);
+				}
 
-				/*
-				 * We're adding a trigger to both tables, so the lock level
-				 * here should sensibly reflect that.
-				 */
-				refrel = heap_open(con->refrelid, ShareRowExclusiveLock);
+				refrel = heap_open(con->refrelid, RowShareLock);
 
 				validateForeignKeyConstraint(fkconstraint->conname, rel, refrel,
 											 con->refindid,
@@ -5529,7 +5527,14 @@ ATAddForeignKeyConstraint(AlteredTableInfo *tab, Relation rel,
 	Oid			indexOid;
 	Oid			constrOid;
 
-	pkrel = heap_openrv(fkconstraint->pktable, lockmode);
+	/*
+	 * 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, AccessExclusiveLock);
 
 	/*
 	 * Validity checks (permission checks wait till we have the column

src/backend/commands/trigger.c

@@ -144,14 +144,7 @@ CreateTrigger(CreateTrigStmt *stmt, const char *queryString,
 	ObjectAddress myself,
 				referenced;
 
-	/*
-	 * ShareRowExclusiveLock is sufficient to prevent concurrent write
-	 * activity to the relation, and thus to lock out any operations that
-	 * might want to fire triggers on the relation.  If we had ON SELECT
-	 * triggers we would need to take an AccessExclusiveLock to add one of
-	 * those, just as we do with ON SELECT rules.
-	 */
-	rel = heap_openrv(stmt->relation, ShareRowExclusiveLock);
+	rel = heap_openrv(stmt->relation, AccessExclusiveLock);
 
 	/*
 	 * Triggers must be on tables or views, and there are additional
@@ -481,7 +474,7 @@ CreateTrigger(CreateTrigStmt *stmt, const char *queryString,
 	 * can skip this for internally generated triggers, since the name
 	 * modification above should be sufficient.
 	 *
-	 * NOTE that this is cool only because we have ShareRowExclusiveLock on
+	 * NOTE that this is cool only because we have AccessExclusiveLock on
 	 * the relation, so the trigger set won't be changing underneath us.
 	 */
 	if (!isInternal)
@@ -1085,14 +1078,11 @@ RemoveTriggerById(Oid trigOid)
 		elog(ERROR, "could not find tuple for trigger %u", trigOid);
 
 	/*
-	 * Open and lock the relation the trigger belongs to.  As in
-	 * CreateTrigger, this is sufficient to lock out all operations that could
-	 * fire or add triggers; but it would need to be revisited if we had ON
-	 * SELECT triggers.
+	 * Open and exclusive-lock the relation the trigger belongs to.
 	 */
 	relid = ((Form_pg_trigger) GETSTRUCT(tup))->tgrelid;
 
-	rel = heap_open(relid, ShareRowExclusiveLock);
+	rel = heap_open(relid, AccessExclusiveLock);
 
 	if (rel->rd_rel->relkind != RELKIND_RELATION &&
 		rel->rd_rel->relkind != RELKIND_VIEW)

src/backend/rewrite/rewriteDefine.c

@@ -238,14 +238,12 @@ DefineQueryRewrite(char *rulename,
 	/*
 	 * If we are installing an ON SELECT rule, we had better grab
 	 * AccessExclusiveLock to ensure no SELECTs are currently running on the
-	 * event relation.	For other types of rules, it is sufficient to grab
-	 * ShareRowExclusiveLock to lock out insert/update/delete actions and to
-	 * ensure that we lock out current CREATE RULE statements.
+	 * event relation. For other types of rules, it would be sufficient to
+	 * grab ShareRowExclusiveLock to lock out insert/update/delete actions and
+	 * to ensure that we lock out current CREATE RULE statements; but because
+	 * of race conditions in access to catalog entries, we can't do that yet.
 	 */
-	if (event_type == CMD_SELECT)
-		event_relation = heap_open(event_relid, AccessExclusiveLock);
-	else
-		event_relation = heap_open(event_relid, ShareRowExclusiveLock);
+	event_relation = heap_open(event_relid, AccessExclusiveLock);
 
 	/*
 	 * Verify relation is of a type that rules can sensibly be applied to.