The result of a FULL or RIGHT join can't be assumed to be sorted by the

left input's sorting, because null rows may be inserted at various points.
Per report from Ferenc Lutischá¸n.

src/backend/optimizer/path/joinpath.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/joinpath.c,v 1.91 2004/12/31 22:00:04 pgsql Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/joinpath.c,v 1.92 2005/01/23 02:21:26 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -271,7 +271,8 @@ sort_inner_and_outer(Query *root,
 												   cur_mergeclauses,
 												   innerrel);
 		/* Build pathkeys representing output sort order. */
-		merge_pathkeys = build_join_pathkeys(root, joinrel, outerkeys);
+		merge_pathkeys = build_join_pathkeys(root, joinrel, jointype,
+											 outerkeys);
 
 		/*
 		 * And now we can make the path.
@@ -431,7 +432,7 @@ match_unsorted_outer(Query *root,
 		 * as a nestloop, and even if some of the mergeclauses are
 		 * implemented by qpquals rather than as true mergeclauses):
 		 */
-		merge_pathkeys = build_join_pathkeys(root, joinrel,
+		merge_pathkeys = build_join_pathkeys(root, joinrel, jointype,
 											 outerpath->pathkeys);
 
 		if (nestjoinOK)

src/backend/optimizer/path/pathkeys.c

@@ -11,7 +11,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.63 2004/12/31 22:00:04 pgsql Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.64 2005/01/23 02:21:26 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -858,7 +858,12 @@ build_subquery_pathkeys(Query *root, RelOptInfo *rel, Query *subquery)
  *	  vars they were joined with; furthermore, it doesn't matter what kind
  *	  of join algorithm is actually used.
  *
+ *	  EXCEPTION: in a FULL or RIGHT join, we cannot treat the result as
+ *	  having the outer path's path keys, because null lefthand rows may be
+ *	  inserted at random points.  It must be treated as unsorted.
+ *
  * 'joinrel' is the join relation that paths are being formed for
+ * 'jointype' is the join type (inner, left, full, etc)
  * 'outer_pathkeys' is the list of the current outer path's path keys
  *
  * Returns the list of new path keys.
@@ -866,8 +871,12 @@ build_subquery_pathkeys(Query *root, RelOptInfo *rel, Query *subquery)
 List *
 build_join_pathkeys(Query *root,
 					RelOptInfo *joinrel,
+					JoinType jointype,
 					List *outer_pathkeys)
 {
+	if (jointype == JOIN_FULL || jointype == JOIN_RIGHT)
+		return NIL;
+
 	/*
 	 * This used to be quite a complex bit of code, but now that all
 	 * pathkey sublists start out life canonicalized, we don't have to do

src/include/optimizer/paths.h

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/optimizer/paths.h,v 1.77 2004/12/31 22:03:36 pgsql Exp $
+ * $PostgreSQL: pgsql/src/include/optimizer/paths.h,v 1.78 2005/01/23 02:21:36 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -114,6 +114,7 @@ extern List *build_subquery_pathkeys(Query *root, RelOptInfo *rel,
 						Query *subquery);
 extern List *build_join_pathkeys(Query *root,
 					RelOptInfo *joinrel,
+					JoinType jointype,
 					List *outer_pathkeys);
 extern List *make_pathkeys_for_sortclauses(List *sortclauses,
 							  List *tlist);