Do not push expressions having mutable functions to the remote server.

If an expression that includes a mutable function is sent on the
remote server via remote query its result may not be stable. Instead,
check if the expression has a mutable function, if yes do not send
that expression to the remote MySQL server.

Reported on GitHub as issue #226 by user robins.

FDW-411, Suraj Kharage, reviewed by Vaibhav Dalvi.

Author: jeevanladhe

deparse.c

@@ -1871,6 +1871,16 @@ mysql_is_foreign_expr(PlannerInfo *root, RelOptInfo *baserel, Expr *expr,
 	Assert(loc_cxt.collation == InvalidOid);
 	Assert(loc_cxt.state == FDW_COLLATE_NONE);
 
+	/*
+	 * An expression which includes any mutable functions can't be sent over
+	 * because its result is not stable.  For example, sending now() remote
+	 * side could cause confusion from clock offsets.  Future versions might
+	 * be able to make this choice with more granularity.  (We check this last
+	 * because it requires a lot of expensive catalog lookups.)
+	 */
+	if (contain_mutable_functions((Node *) expr))
+		return false;
+
 	/* OK to evaluate on the remote server */
 	return true;
 }

expected/select.out

@@ -1402,6 +1402,35 @@ SELECT c1, sum(c7) FROM f_test_tbl1 t1
 (14 rows)
 
 ALTER SERVER mysql_svr options (SET use_remote_estimate 'false');
+-- FDW-411: Volatile/immutable functions should not get pushed down to remote
+-- MySQL server.
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT c1, c2, c3 FROM f_test_tbl1 WHERE pg_catalog.timeofday() IS NOT NULL
+  ORDER BY 1 limit 5;
+                                        QUERY PLAN                                        
+------------------------------------------------------------------------------------------
+ Limit
+   Output: c1, c2, c3
+   ->  Sort
+         Output: c1, c2, c3
+         Sort Key: f_test_tbl1.c1
+         ->  Foreign Scan on public.f_test_tbl1
+               Output: c1, c2, c3
+               Filter: (timeofday() IS NOT NULL)
+               Remote query: SELECT `c1`, `c2`, `c3` FROM `mysql_fdw_regress`.`test_tbl1`
+(9 rows)
+
+SELECT c1, c2, c3 FROM f_test_tbl1 WHERE pg_catalog.timeofday() IS NOT NULL
+  ORDER BY 1 limit 5;
+ c1  |  c2  |    c3    
+-----+------+----------
+ 100 | EMP1 | ADMIN
+ 200 | EMP2 | SALESMAN
+ 300 | EMP3 | SALESMAN
+ 400 | EMP4 | MANAGER
+ 500 | EMP5 | SALESMAN
+(5 rows)
+
 -- Cleanup
 DROP TABLE l_test_tbl1;
 DROP TABLE l_test_tbl2;

sql/select.sql

@@ -411,6 +411,14 @@ SELECT c1, sum(c7) FROM f_test_tbl1 t1
   ORDER BY 1,2;
 ALTER SERVER mysql_svr options (SET use_remote_estimate 'false');
 
+-- FDW-411: Volatile/immutable functions should not get pushed down to remote
+-- MySQL server.
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT c1, c2, c3 FROM f_test_tbl1 WHERE pg_catalog.timeofday() IS NOT NULL
+  ORDER BY 1 limit 5;
+SELECT c1, c2, c3 FROM f_test_tbl1 WHERE pg_catalog.timeofday() IS NOT NULL
+  ORDER BY 1 limit 5;
+
 -- Cleanup
 DROP TABLE l_test_tbl1;
 DROP TABLE l_test_tbl2;