Don't try to optimize EXISTS subqueries with empty FROM-lists: we need to

form a join and that case doesn't have anything to join to.  (We could
probably make it work if we didn't pull up the subquery, but it seems to
me that the case isn't worth extra code.)  Per report from Greg Stark.

src/backend/optimizer/plan/subselect.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/subselect.c,v 1.142 2008/10/21 20:42:53 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/subselect.c,v 1.143 2008/12/08 00:16:09 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1097,6 +1097,12 @@ convert_EXISTS_sublink_to_join(PlannerInfo *root, SubLink *sublink,
 	if (!simplify_EXISTS_query(subselect))
 		return false;
 
+	/*
+	 * The subquery must have a nonempty jointree, else we won't have a join.
+	 */
+	if (subselect->jointree->fromlist == NIL)
+		return false;
+
 	/*
 	 * Separate out the WHERE clause.  (We could theoretically also remove
 	 * top-level plain JOIN/ON clauses, but it's probably not worth the
@@ -1180,6 +1186,7 @@ convert_EXISTS_sublink_to_join(PlannerInfo *root, SubLink *sublink,
 	/* Identify all the rels syntactically within the subselect */
 	subselect_varnos = get_relids_in_jointree((Node *) subselect->jointree,
 											  true);
+	Assert(!bms_is_empty(subselect_varnos));
 	Assert(bms_is_subset(right_varnos, subselect_varnos));
 
 	/* Now we can attach the modified subquery rtable to the parent */