diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index cc6a1cdf40..1f979dd409 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.109 2003/11/29 19:51:50 pgsql Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.110 2003/12/17 17:07:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -545,6 +545,8 @@ make_one_rel_by_joins(Query *root, int levels_needed, List *initial_rels)
 	/*
 	 * We should have a single rel at the final level.
 	 */
+	if (joinitems[levels_needed] == NIL)
+		elog(ERROR, "failed to build any %d-way joins", levels_needed);
 	Assert(length(joinitems[levels_needed]) == 1);
 
 	rel = (RelOptInfo *) lfirst(joinitems[levels_needed]);
diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c
index 1ac9a3064b..26ff2e5758 100644
--- a/src/backend/optimizer/path/joinrels.c
+++ b/src/backend/optimizer/path/joinrels.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/joinrels.c,v 1.64 2003/11/29 19:51:50 pgsql Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/joinrels.c,v 1.65 2003/12/17 17:07:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -226,7 +226,24 @@ make_rels_by_joins(Query *root, int level, List **joinrels)
 			}
 		}
 
-		if (result_rels == NIL)
+		/*----------
+		 * When IN clauses are involved, there may be no legal way to make
+		 * an N-way join for some values of N.  For example consider
+		 *
+		 * SELECT ... FROM t1 WHERE
+		 *   x IN (SELECT ... FROM t2,t3 WHERE ...) AND
+		 *   y IN (SELECT ... FROM t4,t5 WHERE ...)
+		 *
+		 * We will flatten this query to a 5-way join problem, but there are
+		 * no 4-way joins that make_join_rel() will consider legal.  We have
+		 * to accept failure at level 4 and go on to discover a workable
+		 * bushy plan at level 5.
+		 *
+		 * However, if there are no IN clauses then make_join_rel() should
+		 * never fail, and so the following sanity check is useful.
+		 *----------
+		 */
+		if (result_rels == NIL && root->in_info_list == NIL)
 			elog(ERROR, "failed to build any %d-way joins", level);
 	}