diff --git a/src/backend/pgxc/plan/planner.c b/src/backend/pgxc/plan/planner.c
index 420a531..87a72e8 100644
--- a/src/backend/pgxc/plan/planner.c
+++ b/src/backend/pgxc/plan/planner.c
@@ -1622,7 +1622,7 @@ pgxc_shippability_walker(Node *node, Shippability_context *sc_context)
 			 * functions to the Datanode. We need a better way to see what
 			 * can be shipped to the Datanode and what can not be.
 			 */
-			if (!is_immutable_func(funcexpr->funcid))
+			if (func_volatile(funcexpr->funcid) != PROVOLATILE_IMMUTABLE)
 				pgxc_set_shippability_reason(sc_context, SS_UNSHIPPABLE_EXPR);
 		}
 		break;
@@ -1639,7 +1639,8 @@ pgxc_shippability_walker(Node *node, Shippability_context *sc_context)
 			OpExpr *op_expr = (OpExpr *)node;
 			Oid		opfuncid = OidIsValid(op_expr->opfuncid) ?
 								op_expr->opfuncid : get_opcode(op_expr->opno);
-			if (!OidIsValid(opfuncid) || !is_immutable_func(opfuncid))
+			if (!OidIsValid(opfuncid) ||
+				func_volatile(opfuncid) != PROVOLATILE_IMMUTABLE)
 				pgxc_set_shippability_reason(sc_context, SS_UNSHIPPABLE_EXPR);
 		}
 		break;
@@ -1653,7 +1654,8 @@ pgxc_shippability_walker(Node *node, Shippability_context *sc_context)
 			ScalarArrayOpExpr *sao_expr = (ScalarArrayOpExpr *)node;
 			Oid		opfuncid = OidIsValid(sao_expr->opfuncid) ?
 								sao_expr->opfuncid : get_opcode(sao_expr->opno);
-			if (!OidIsValid(opfuncid) || !is_immutable_func(opfuncid))
+			if (!OidIsValid(opfuncid) ||
+				func_volatile(opfuncid) != PROVOLATILE_IMMUTABLE)
 				pgxc_set_shippability_reason(sc_context, SS_UNSHIPPABLE_EXPR);
 		}
 		break;
@@ -1786,7 +1788,7 @@ pgxc_shippability_walker(Node *node, Shippability_context *sc_context)
 			 * be shipped to the datanodes.
 			 */
 			if (sc_context->sc_for_expr ||
-				!is_immutable_func(winf->winfnoid))
+				func_volatile(winf->winfnoid) != PROVOLATILE_IMMUTABLE)
 				pgxc_set_shippability_reason(sc_context, SS_UNSHIPPABLE_EXPR);
 		}
 		break;
diff --git a/src/backend/pgxc/pool/postgresql_fdw.c b/src/backend/pgxc/pool/postgresql_fdw.c
index e8bb61b..133e4d3 100644
--- a/src/backend/pgxc/pool/postgresql_fdw.c
+++ b/src/backend/pgxc/pool/postgresql_fdw.c
@@ -31,38 +31,6 @@
 #define DEBUG_FDW
 
 /*
- * Check whether the function is IMMUTABLE.
- */
-bool
-is_immutable_func(Oid funcid)
-{
-	HeapTuple		tp;
-	bool			isnull;
-	Datum			datum;
-
-	tp = SearchSysCache(PROCOID, ObjectIdGetDatum(funcid), 0, 0, 0);
-	if (!HeapTupleIsValid(tp))
-		elog(ERROR, "cache lookup failed for function %u", funcid);
-
-#ifdef DEBUG_FDW
-	/* print function name and its immutability */
-	{
-		char		   *proname;
-		datum = SysCacheGetAttr(PROCOID, tp, Anum_pg_proc_proname, &isnull);
-		proname = pstrdup(DatumGetName(datum)->data);
-		elog(DEBUG1, "func %s(%u) is%s immutable", proname, funcid,
-			(DatumGetChar(datum) == PROVOLATILE_IMMUTABLE) ? "" : " not");
-		pfree(proname);
-	}
-#endif
-
-	datum = SysCacheGetAttr(PROCOID, tp, Anum_pg_proc_provolatile, &isnull);
-	ReleaseSysCache(tp);
-
-	return (DatumGetChar(datum) == PROVOLATILE_IMMUTABLE);
-}
-
-/*
  * Check whether the ExprState node should be evaluated in foreign server.
  *
  * An expression which consists of expressions below will be evaluated in
diff --git a/src/include/pgxc/postgresql_fdw.h b/src/include/pgxc/postgresql_fdw.h
index 57ab2b7..0b75032 100644
--- a/src/include/pgxc/postgresql_fdw.h
+++ b/src/include/pgxc/postgresql_fdw.h
@@ -18,6 +18,5 @@
 #include "postgres.h"
 #include "pgxc/execRemote.h"
 
-bool is_immutable_func(Oid funcid);
 bool pgxc_is_expr_shippable(Expr *node, bool *has_aggs);
 #endif