diff --git a/src/backend/optimizer/plan/subselect.c b/src/backend/optimizer/plan/subselect.c
index cd6b32a77f07c05e59f8a8c3dffe7e530c6b185f..78f6b6eec9b41b2a3bd234079338d479ebcfc865 100644
--- a/src/backend/optimizer/plan/subselect.c
+++ b/src/backend/optimizer/plan/subselect.c
@@ -9,7 +9,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/subselect.c,v 1.148 2009/04/05 19:59:40 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/subselect.c,v 1.150.2.2 2010/01/18 18:17:52 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1369,6 +1369,17 @@ convert_EXISTS_sublink_to_join(PlannerInfo *root, SubLink *sublink,
 	if (has_correlation_in_funcexpr_rte(subselect->rtable))
 		return NULL;
 
+	/*
+	 * Can't flatten if it contains WITH.  (We could arrange to pull up the
+	 * WITH into the parent query's cteList, but that risks changing the
+	 * semantics, since a WITH ought to be executed once per associated query
+	 * call.)  Note that convert_ANY_sublink_to_join doesn't have to reject
+	 * this case, since it just produces a subquery RTE that doesn't have to
+	 * get flattened into the parent query.
+	 */
+	if (subselect->cteList)
+		return NULL;
+
 	/*
 	 * If deeply correlated, don't bother.
 	 */