Ooops ... make_outerjoininfo wasn't actually enforcing the join order

restrictions specified for semijoins in optimizer/README, to wit that
you can't reassociate outer joins into or out of the RHS of a semijoin.
Per report from Heikki.

src/backend/optimizer/plan/initsplan.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.152 2009/05/06 20:31:18 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.153 2009/05/07 20:13:09 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -654,7 +654,8 @@ make_outerjoininfo(PlannerInfo *root,
 		 * lower join's RHS and the lower OJ's join condition is strict, we
 		 * can interchange the ordering of the two OJs; otherwise we must add
 		 * lower OJ's full syntactic relset to min_righthand.  Here, we must
-		 * preserve ordering anyway if the lower OJ is an antijoin.
+		 * preserve ordering anyway if either the current join is a semijoin,
+		 * or the lower OJ is an antijoin.
 		 *
 		 * Here, we have to consider that "our join condition" includes any
 		 * clauses that syntactically appeared above the lower OJ and below
@@ -670,6 +671,7 @@ make_outerjoininfo(PlannerInfo *root,
 		if (bms_overlap(right_rels, otherinfo->syn_righthand))
 		{
 			if (bms_overlap(clause_relids, otherinfo->syn_righthand) ||
+				jointype == JOIN_SEMI ||
 				otherinfo->jointype == JOIN_ANTI ||
 				!otherinfo->lhs_strict || otherinfo->delay_upper_joins)
 			{