diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
index b0e8c94dfc3..a244300463c 100644
--- a/src/backend/optimizer/path/joinpath.c
+++ b/src/backend/optimizer/path/joinpath.c
@@ -25,6 +25,7 @@
 #include "optimizer/paths.h"
 #include "optimizer/placeholder.h"
 #include "optimizer/planmain.h"
+#include "utils/lsyscache.h"
 #include "utils/typcache.h"
 
 /* Hook for plugins to get control in add_paths_to_joinrel() */
@@ -2266,6 +2267,20 @@ hash_inner_and_outer(PlannerInfo *root,
 		if (!clause_sides_match_join(restrictinfo, outerrel, innerrel))
 			continue;			/* no good for these input relations */
 
+		/*
+		 * If clause has the form "inner op outer", check if its operator has
+		 * valid commutator.  This is necessary because hashclauses in this
+		 * form will get commuted in createplan.c to put the outer var on the
+		 * left (see get_switched_clauses).  This probably shouldn't ever
+		 * fail, since hashable operators ought to have commutators, but be
+		 * paranoid.
+		 *
+		 * The clause being hashjoinable indicates that it's an OpExpr.
+		 */
+		if (!restrictinfo->outer_is_left &&
+			!OidIsValid(get_commutator(castNode(OpExpr, restrictinfo->clause)->opno)))
+			continue;
+
 		hashclauses = lappend(hashclauses, restrictinfo);
 	}
 
@@ -2540,6 +2555,23 @@ select_mergejoin_clauses(PlannerInfo *root,
 			continue;			/* no good for these input relations */
 		}
 
+		/*
+		 * If clause has the form "inner op outer", check if its operator has
+		 * valid commutator.  This is necessary because mergejoin clauses in
+		 * this form will get commuted in createplan.c to put the outer var on
+		 * the left (see get_switched_clauses).  This probably shouldn't ever
+		 * fail, since mergejoinable operators ought to have commutators, but
+		 * be paranoid.
+		 *
+		 * The clause being mergejoinable indicates that it's an OpExpr.
+		 */
+		if (!restrictinfo->outer_is_left &&
+			!OidIsValid(get_commutator(castNode(OpExpr, restrictinfo->clause)->opno)))
+		{
+			have_nonmergeable_joinclause = true;
+			continue;
+		}
+
 		/*
 		 * Insist that each side have a non-redundant eclass.  This
 		 * restriction is needed because various bits of the planner expect
diff --git a/src/test/regress/expected/equivclass.out b/src/test/regress/expected/equivclass.out
index 126f7047fed..a328164fe0f 100644
--- a/src/test/regress/expected/equivclass.out
+++ b/src/test/regress/expected/equivclass.out
@@ -451,3 +451,51 @@ explain (costs off)  -- this should not require a sort
    Filter: (f1 = 'foo'::name)
 (2 rows)
 
+--
+-- test handling of merge/hash clauses that do not have valid commutators
+--
+-- There are not (and should not be) any such operators built into Postgres
+-- that are mergejoinable or hashable but have no commutators; so we leverage
+-- the alias type 'int8alias1' created in this file to conduct the tests.
+-- That's why this test is included here rather than in join.sql.
+begin;
+create table tbl_nocom(a int8, b int8alias1);
+-- check that non-commutable merge clauses do not lead to error
+set enable_hashjoin to off;
+set enable_mergejoin to on;
+explain (costs off)
+select * from tbl_nocom t1 full join tbl_nocom t2 on t2.a = t1.b;
+              QUERY PLAN              
+--------------------------------------
+ Merge Full Join
+   Merge Cond: (t2.a = t1.b)
+   ->  Sort
+         Sort Key: t2.a
+         ->  Seq Scan on tbl_nocom t2
+   ->  Sort
+         Sort Key: t1.b USING <
+         ->  Seq Scan on tbl_nocom t1
+(8 rows)
+
+-- check that non-commutable hash clauses do not lead to error
+alter operator = (int8, int8alias1) set (hashes);
+alter operator family integer_ops using hash add
+  operator 1 = (int8, int8alias1);
+create function hashint8alias1(int8alias1) returns int
+  strict immutable language internal as 'hashint8';
+alter operator family integer_ops using hash add
+  function 1 hashint8alias1(int8alias1);
+set enable_hashjoin to on;
+set enable_mergejoin to off;
+explain (costs off)
+select * from tbl_nocom t1 full join tbl_nocom t2 on t2.a = t1.b;
+              QUERY PLAN              
+--------------------------------------
+ Hash Full Join
+   Hash Cond: (t2.a = t1.b)
+   ->  Seq Scan on tbl_nocom t2
+   ->  Hash
+         ->  Seq Scan on tbl_nocom t1
+(5 rows)
+
+abort;
diff --git a/src/test/regress/sql/equivclass.sql b/src/test/regress/sql/equivclass.sql
index 247b0a31055..28ed7910d01 100644
--- a/src/test/regress/sql/equivclass.sql
+++ b/src/test/regress/sql/equivclass.sql
@@ -269,3 +269,37 @@ create temp view overview as
   select f1::information_schema.sql_identifier as sqli, f2 from undername;
 explain (costs off)  -- this should not require a sort
   select * from overview where sqli = 'foo' order by sqli;
+
+--
+-- test handling of merge/hash clauses that do not have valid commutators
+--
+
+-- There are not (and should not be) any such operators built into Postgres
+-- that are mergejoinable or hashable but have no commutators; so we leverage
+-- the alias type 'int8alias1' created in this file to conduct the tests.
+-- That's why this test is included here rather than in join.sql.
+
+begin;
+
+create table tbl_nocom(a int8, b int8alias1);
+
+-- check that non-commutable merge clauses do not lead to error
+set enable_hashjoin to off;
+set enable_mergejoin to on;
+explain (costs off)
+select * from tbl_nocom t1 full join tbl_nocom t2 on t2.a = t1.b;
+
+-- check that non-commutable hash clauses do not lead to error
+alter operator = (int8, int8alias1) set (hashes);
+alter operator family integer_ops using hash add
+  operator 1 = (int8, int8alias1);
+create function hashint8alias1(int8alias1) returns int
+  strict immutable language internal as 'hashint8';
+alter operator family integer_ops using hash add
+  function 1 hashint8alias1(int8alias1);
+set enable_hashjoin to on;
+set enable_mergejoin to off;
+explain (costs off)
+select * from tbl_nocom t1 full join tbl_nocom t2 on t2.a = t1.b;
+
+abort;