diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index 379e15a9641..f35372ebbef 100644
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.212.2.2 2007/05/22 01:40:42 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/indxpath.c,v 1.212.2.3 2007/07/31 19:53:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1755,6 +1755,8 @@ find_clauses_for_join(PlannerInfo *root, RelOptInfo *rel,
 	{
 		clause_list = remove_redundant_join_clauses(root,
 													clause_list,
+													outer_relids,
+													rel->relids,
 													isouterjoin);
 	}
 
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 14f1f1a10f4..75b67e8879f 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.217 2006/10/04 00:29:54 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.217.2.1 2007/07/31 19:53:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1398,7 +1398,9 @@ create_nestloop_plan(PlannerInfo *root,
 				select_nonredundant_join_clauses(root,
 												 joinrestrictclauses,
 												 innerpath->indexclauses,
-										 IS_OUTER_JOIN(best_path->jointype));
+									best_path->outerjoinpath->parent->relids,
+									best_path->innerjoinpath->parent->relids,
+									IS_OUTER_JOIN(best_path->jointype));
 		}
 	}
 	else if (IsA(best_path->innerjoinpath, BitmapHeapPath))
@@ -1430,7 +1432,9 @@ create_nestloop_plan(PlannerInfo *root,
 				select_nonredundant_join_clauses(root,
 												 joinrestrictclauses,
 												 bitmapclauses,
-										 IS_OUTER_JOIN(best_path->jointype));
+									best_path->outerjoinpath->parent->relids,
+									best_path->innerjoinpath->parent->relids,
+									IS_OUTER_JOIN(best_path->jointype));
 		}
 	}
 
diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c
index 4d5bffdb7b1..2a3a14ae6b1 100644
--- a/src/backend/optimizer/util/relnode.c
+++ b/src/backend/optimizer/util/relnode.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/util/relnode.c,v 1.83 2006/10/04 00:29:55 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/util/relnode.c,v 1.83.2.1 2007/07/31 19:53:50 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -515,6 +515,8 @@ build_joinrel_restrictlist(PlannerInfo *root,
 	 * omit the redundant clause from the result list.
 	 */
 	result = remove_redundant_join_clauses(root, rlist,
+										   outer_rel->relids,
+										   inner_rel->relids,
 										   IS_OUTER_JOIN(jointype));
 
 	list_free(rlist);
diff --git a/src/backend/optimizer/util/restrictinfo.c b/src/backend/optimizer/util/restrictinfo.c
index 9176ae1680c..ae114f65509 100644
--- a/src/backend/optimizer/util/restrictinfo.c
+++ b/src/backend/optimizer/util/restrictinfo.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/util/restrictinfo.c,v 1.49 2006/10/04 00:29:55 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/util/restrictinfo.c,v 1.49.2.1 2007/07/31 19:53:50 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -36,6 +36,8 @@ static Expr *make_sub_restrictinfos(Expr *clause,
 static RestrictInfo *join_clause_is_redundant(PlannerInfo *root,
 						 RestrictInfo *rinfo,
 						 List *reference_list,
+						 Relids outer_relids,
+						 Relids inner_relids,
 						 bool isouterjoin);
 
 
@@ -546,6 +548,8 @@ extract_actual_join_clauses(List *restrictinfo_list,
  */
 List *
 remove_redundant_join_clauses(PlannerInfo *root, List *restrictinfo_list,
+							  Relids outer_relids,
+							  Relids inner_relids,
 							  bool isouterjoin)
 {
 	List	   *result = NIL;
@@ -572,7 +576,9 @@ remove_redundant_join_clauses(PlannerInfo *root, List *restrictinfo_list,
 		RestrictInfo *prevrinfo;
 
 		/* is it redundant with any prior clause? */
-		prevrinfo = join_clause_is_redundant(root, rinfo, result, isouterjoin);
+		prevrinfo = join_clause_is_redundant(root, rinfo, result,
+											 outer_relids, inner_relids,
+											 isouterjoin);
 		if (prevrinfo == NULL)
 		{
 			/* no, so add it to result list */
@@ -608,6 +614,8 @@ List *
 select_nonredundant_join_clauses(PlannerInfo *root,
 								 List *restrictinfo_list,
 								 List *reference_list,
+								 Relids outer_relids,
+								 Relids inner_relids,
 								 bool isouterjoin)
 {
 	List	   *result = NIL;
@@ -618,7 +626,9 @@ select_nonredundant_join_clauses(PlannerInfo *root,
 		RestrictInfo *rinfo = (RestrictInfo *) lfirst(item);
 
 		/* drop it if redundant with any reference clause */
-		if (join_clause_is_redundant(root, rinfo, reference_list, isouterjoin) != NULL)
+		if (join_clause_is_redundant(root, rinfo, reference_list,
+									 outer_relids, inner_relids,
+									 isouterjoin) != NULL)
 			continue;
 
 		/* otherwise, add it to result list */
@@ -651,6 +661,12 @@ select_nonredundant_join_clauses(PlannerInfo *root,
  * of the latter, even though they might seem redundant by the pathkey
  * membership test.
  *
+ * Also, we cannot eliminate clauses wherein one side mentions vars from
+ * both relations, as in "WHERE t1.f1 = t2.f1 AND t1.f1 = t1.f2 - t2.f2".
+ * In this example, "t1.f2 - t2.f2" could not have been computed at all
+ * before forming the join of t1 and t2, so it certainly wasn't constrained
+ * earlier.
+ *
  * Weird special case: if we have two clauses that seem redundant
  * except one is pushed down into an outer join and the other isn't,
  * then they're not really redundant, because one constrains the
@@ -660,6 +676,8 @@ static RestrictInfo *
 join_clause_is_redundant(PlannerInfo *root,
 						 RestrictInfo *rinfo,
 						 List *reference_list,
+						 Relids outer_relids,
+						 Relids inner_relids,
 						 bool isouterjoin)
 {
 	ListCell   *refitem;
@@ -681,6 +699,14 @@ join_clause_is_redundant(PlannerInfo *root,
 			bms_is_empty(rinfo->right_relids))
 			return NULL;		/* var = const, so not redundant */
 
+		/* check for either side mentioning both rels */
+		if (bms_overlap(rinfo->left_relids, outer_relids) &&
+			bms_overlap(rinfo->left_relids, inner_relids))
+			return NULL;		/* clause LHS uses both, so not redundant */
+		if (bms_overlap(rinfo->right_relids, outer_relids) &&
+			bms_overlap(rinfo->right_relids, inner_relids))
+			return NULL;		/* clause RHS uses both, so not redundant */
+
 		cache_mergeclause_pathkeys(root, rinfo);
 
 		foreach(refitem, reference_list)
diff --git a/src/include/optimizer/restrictinfo.h b/src/include/optimizer/restrictinfo.h
index db24c4989db..c9639a2c059 100644
--- a/src/include/optimizer/restrictinfo.h
+++ b/src/include/optimizer/restrictinfo.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/optimizer/restrictinfo.h,v 1.38 2006/10/04 00:30:09 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/optimizer/restrictinfo.h,v 1.38.2.1 2007/07/31 19:53:50 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -34,10 +34,14 @@ extern void extract_actual_join_clauses(List *restrictinfo_list,
 							List **otherquals);
 extern List *remove_redundant_join_clauses(PlannerInfo *root,
 							  List *restrictinfo_list,
+							  Relids outer_relids,
+							  Relids inner_relids,
 							  bool isouterjoin);
 extern List *select_nonredundant_join_clauses(PlannerInfo *root,
 								 List *restrictinfo_list,
 								 List *reference_list,
+								 Relids outer_relids,
+								 Relids inner_relids,
 								 bool isouterjoin);
 
 #endif   /* RESTRICTINFO_H */
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index 8b6716def81..3b7bdd9cbb3 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -2250,3 +2250,19 @@ WHERE d.f1 IS NULL;
  9999
 (3 rows)
 
+--
+-- regression test for problems of the sort depicted in bug #3494
+--
+create temp table tt5(f1 int, f2 int);
+create temp table tt6(f1 int, f2 int);
+insert into tt5 values(1, 10);
+insert into tt5 values(1, 11);
+insert into tt6 values(1, 9);
+insert into tt6 values(1, 2);
+insert into tt6 values(2, 9);
+select * from tt5,tt6 where tt5.f1 = tt6.f1 and tt5.f1 = tt5.f2 - tt6.f2;
+ f1 | f2 | f1 | f2 
+----+----+----+----
+  1 | 10 |  1 |  9
+(1 row)
+
diff --git a/src/test/regress/expected/join_1.out b/src/test/regress/expected/join_1.out
index 8e7e4de0141..f1bf4b9af26 100644
--- a/src/test/regress/expected/join_1.out
+++ b/src/test/regress/expected/join_1.out
@@ -2250,3 +2250,19 @@ WHERE d.f1 IS NULL;
  9999
 (3 rows)
 
+--
+-- regression test for problems of the sort depicted in bug #3494
+--
+create temp table tt5(f1 int, f2 int);
+create temp table tt6(f1 int, f2 int);
+insert into tt5 values(1, 10);
+insert into tt5 values(1, 11);
+insert into tt6 values(1, 9);
+insert into tt6 values(1, 2);
+insert into tt6 values(2, 9);
+select * from tt5,tt6 where tt5.f1 = tt6.f1 and tt5.f1 = tt5.f2 - tt6.f2;
+ f1 | f2 | f1 | f2 
+----+----+----+----
+  1 | 10 |  1 |  9
+(1 row)
+
diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index d3433effd11..ad8957cd6e3 100644
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -424,3 +424,19 @@ LEFT JOIN (
         WHERE c.f1 IS NULL
 ) AS d ON (a.f1 = d.f1)
 WHERE d.f1 IS NULL;
+
+--
+-- regression test for problems of the sort depicted in bug #3494
+--
+
+create temp table tt5(f1 int, f2 int);
+create temp table tt6(f1 int, f2 int);
+
+insert into tt5 values(1, 10);
+insert into tt5 values(1, 11);
+
+insert into tt6 values(1, 9);
+insert into tt6 values(1, 2);
+insert into tt6 values(2, 9);
+
+select * from tt5,tt6 where tt5.f1 = tt6.f1 and tt5.f1 = tt5.f2 - tt6.f2;