diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index 505ae0af85..2c9983004a 100644
--- a/src/backend/optimizer/util/clauses.c
+++ b/src/backend/optimizer/util/clauses.c
@@ -3655,6 +3655,59 @@ eval_const_expressions_mutator(Node *node,
 													  context);
 			}
 			break;
+		case T_Var:
+			if (context->root && context->root->parse->rtable)
+			{
+				Var		   *var;
+				Query	   *query;
+				RangeTblEntry *pointedNode;
+
+				var = (Var *)node;
+				query = context->root->parse;
+
+				if (var->varlevelsup != 0 || var->varattno != 1)
+					break;
+
+				pointedNode = list_nth(query->rtable, var->varno - 1);
+				Assert(IsA(pointedNode, RangeTblEntry));
+
+				if (pointedNode->rtekind == RTE_FUNCTION && list_length(pointedNode->functions) == 1)
+				{
+					Form_pg_type type_form;
+					Node	   *result;
+					RangeTblFunction *tblFunction = linitial_node(RangeTblFunction, pointedNode->functions);
+					FuncExpr   *expr = (FuncExpr *) tblFunction->funcexpr;
+					List	   *args = expr->args;
+					HeapTuple type_tuple = SearchSysCache1(TYPEOID, ObjectIdGetDatum(expr->funcresulttype));
+
+					if (!HeapTupleIsValid(type_tuple))
+						elog(ERROR, "cache lookup failed for type %u", expr->funcresulttype);
+
+					type_form = (Form_pg_type) GETSTRUCT(type_tuple);
+
+					if (type_form->typtype != TYPTYPE_BASE)
+					{
+						ReleaseSysCache(type_tuple);
+						break;
+					}
+
+					result = simplify_function(expr->funcid,
+											   expr->funcresulttype,
+											   exprTypmod(expr),
+											   expr->funccollid,
+											   expr->inputcollid,
+											   &args,
+											   expr->funcvariadic,
+											   true,
+											   false,
+											   context);
+
+					ReleaseSysCache(type_tuple);
+
+					if (result) /* successfully simplified it */
+						return (Node *) result;
+			}
+			break;
 		default:
 			break;
 	}
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index cbc882d47b..eb92f556d8 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -3021,23 +3021,23 @@ select * from
   int4(sin(1)) q1,
   int4(sin(0)) q2
 where q1 = thousand or q2 = thousand;
-                               QUERY PLAN                               
-------------------------------------------------------------------------
- Hash Join
-   Hash Cond: (tenk1.twothousand = int4_tbl.f1)
+                               QUERY PLAN                                
+-------------------------------------------------------------------------
+ Nested Loop
    ->  Nested Loop
-         ->  Nested Loop
-               ->  Function Scan on q1
-               ->  Function Scan on q2
-         ->  Bitmap Heap Scan on tenk1
-               Recheck Cond: ((q1.q1 = thousand) OR (q2.q2 = thousand))
-               ->  BitmapOr
-                     ->  Bitmap Index Scan on tenk1_thous_tenthous
-                           Index Cond: (q1.q1 = thousand)
-                     ->  Bitmap Index Scan on tenk1_thous_tenthous
-                           Index Cond: (q2.q2 = thousand)
-   ->  Hash
-         ->  Seq Scan on int4_tbl
+         ->  Hash Join
+               Hash Cond: (tenk1.twothousand = int4_tbl.f1)
+               ->  Bitmap Heap Scan on tenk1
+                     Recheck Cond: ((1 = thousand) OR (0 = thousand))
+                     ->  BitmapOr
+                           ->  Bitmap Index Scan on tenk1_thous_tenthous
+                                 Index Cond: (1 = thousand)
+                           ->  Bitmap Index Scan on tenk1_thous_tenthous
+                                 Index Cond: (0 = thousand)
+               ->  Hash
+                     ->  Seq Scan on int4_tbl
+         ->  Function Scan on q1
+   ->  Function Scan on q2
 (15 rows)
 
 explain (costs off)
@@ -3046,20 +3046,20 @@ select * from
   int4(sin(1)) q1,
   int4(sin(0)) q2
 where thousand = (q1 + q2);
-                          QUERY PLAN                          
---------------------------------------------------------------
- Hash Join
-   Hash Cond: (tenk1.twothousand = int4_tbl.f1)
+                            QUERY PLAN                             
+-------------------------------------------------------------------
+ Nested Loop
    ->  Nested Loop
-         ->  Nested Loop
-               ->  Function Scan on q1
-               ->  Function Scan on q2
-         ->  Bitmap Heap Scan on tenk1
-               Recheck Cond: (thousand = (q1.q1 + q2.q2))
-               ->  Bitmap Index Scan on tenk1_thous_tenthous
-                     Index Cond: (thousand = (q1.q1 + q2.q2))
-   ->  Hash
-         ->  Seq Scan on int4_tbl
+         ->  Hash Join
+               Hash Cond: (tenk1.twothousand = int4_tbl.f1)
+               ->  Bitmap Heap Scan on tenk1
+                     Recheck Cond: (thousand = 1)
+                     ->  Bitmap Index Scan on tenk1_thous_tenthous
+                           Index Cond: (thousand = 1)
+               ->  Hash
+                     ->  Seq Scan on int4_tbl
+         ->  Function Scan on q1
+   ->  Function Scan on q2
 (12 rows)
 
 --