diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c
index c16b2fdf514e19707fd4d8c609b61cd93516b4f6..3c99c8ce49df29e7cd0a6fb36c3f3c41ec27e93d 100644
--- a/src/backend/optimizer/path/joinrels.c
+++ b/src/backend/optimizer/path/joinrels.c
@@ -470,11 +470,30 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
 		{
 			/*
 			 * Otherwise, the proposed join overlaps the RHS but isn't a valid
-			 * implementation of this SJ.  It might still be a legal join,
-			 * however, if we're allowed to associate it into the RHS of this
-			 * SJ.  That means this SJ must be a LEFT join (not SEMI or ANTI,
-			 * and certainly not FULL) and the proposed join must not overlap
-			 * the LHS.
+			 * implementation of this SJ.  But don't panic quite yet: the RHS
+			 * violation might have occurred previously, in one or both input
+			 * relations, in which case we must have previously decided that
+			 * it was OK to commute some other SJ with this one.  If we need
+			 * to perform this join to finish building up the RHS, rejecting
+			 * it could lead to not finding any plan at all.  (This can occur
+			 * because of the heuristics elsewhere in this file that postpone
+			 * clauseless joins: we might not consider doing a clauseless join
+			 * within the RHS until after we've performed other, validly
+			 * commutable SJs with one or both sides of the clauseless join.)
+			 * This consideration boils down to the rule that if both inputs
+			 * overlap the RHS, we can allow the join --- they are either
+			 * fully within the RHS, or represent previously-allowed joins to
+			 * rels outside it.
+			 */
+			if (bms_overlap(rel1->relids, sjinfo->min_righthand) &&
+				bms_overlap(rel2->relids, sjinfo->min_righthand))
+				continue;		/* assume valid previous violation of RHS */
+
+			/*
+			 * The proposed join could still be legal, but only if we're
+			 * allowed to associate it into the RHS of this SJ.  That means
+			 * this SJ must be a LEFT join (not SEMI or ANTI, and certainly
+			 * not FULL) and the proposed join must not overlap the LHS.
 			 */
 			if (sjinfo->jointype != JOIN_LEFT ||
 				bms_overlap(joinrelids, sjinfo->min_lefthand))
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index 9b1b4b76ab281e5384c9baf6effaa6b463804952..e082b5b21251f90a1df2279eb4b1622ac4be446c 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -3523,6 +3523,52 @@ select t1.* from
  hi de ho neighbor
 (2 rows)
 
+explain (verbose, costs off)
+select * from
+  text_tbl t1
+  inner join int8_tbl i8
+  on i8.q2 = 456
+  right join text_tbl t2
+  on t1.f1 = 'doh!'
+  left join int4_tbl i4
+  on i8.q1 = i4.f1;
+                       QUERY PLAN                       
+--------------------------------------------------------
+ Nested Loop Left Join
+   Output: t1.f1, i8.q1, i8.q2, t2.f1, i4.f1
+   ->  Seq Scan on public.text_tbl t2
+         Output: t2.f1
+   ->  Materialize
+         Output: i8.q1, i8.q2, i4.f1, t1.f1
+         ->  Nested Loop
+               Output: i8.q1, i8.q2, i4.f1, t1.f1
+               ->  Nested Loop Left Join
+                     Output: i8.q1, i8.q2, i4.f1
+                     Join Filter: (i8.q1 = i4.f1)
+                     ->  Seq Scan on public.int8_tbl i8
+                           Output: i8.q1, i8.q2
+                           Filter: (i8.q2 = 456)
+                     ->  Seq Scan on public.int4_tbl i4
+                           Output: i4.f1
+               ->  Seq Scan on public.text_tbl t1
+                     Output: t1.f1
+                     Filter: (t1.f1 = 'doh!'::text)
+(19 rows)
+
+select * from
+  text_tbl t1
+  inner join int8_tbl i8
+  on i8.q2 = 456
+  right join text_tbl t2
+  on t1.f1 = 'doh!'
+  left join int4_tbl i4
+  on i8.q1 = i4.f1;
+  f1  | q1  | q2  |        f1         | f1 
+------+-----+-----+-------------------+----
+ doh! | 123 | 456 | doh!              |   
+ doh! | 123 | 456 | hi de ho neighbor |   
+(2 rows)
+
 --
 -- test ability to push constants through outer join clauses
 --
diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index 39d0f561cd0624b873fbfda0cb7403b1708f8fc1..e2f341acbf2fc5176663be4ca51e82e964465e68 100644
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -1092,6 +1092,25 @@ select t1.* from
   left join int4_tbl i4
   on (i8.q2 = i4.f1);
 
+explain (verbose, costs off)
+select * from
+  text_tbl t1
+  inner join int8_tbl i8
+  on i8.q2 = 456
+  right join text_tbl t2
+  on t1.f1 = 'doh!'
+  left join int4_tbl i4
+  on i8.q1 = i4.f1;
+
+select * from
+  text_tbl t1
+  inner join int8_tbl i8
+  on i8.q2 = 456
+  right join text_tbl t2
+  on t1.f1 = 'doh!'
+  left join int4_tbl i4
+  on i8.q1 = i4.f1;
+
 --
 -- test ability to push constants through outer join clauses
 --