Restrict LIMIT/OFFSET value to either be a constant or a parameter.

Not all expressions are supported on MySQL. Thus restrict to only constant and placeholder parameters in LIMIT and OFFSET clauses. This avoids errors while preparing a MySQL query. Suraj Kharage and Jeevan Chalke.
EnterpriseDB · May 16, 2022 · 7da2699 · 7da2699
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commit 7da2699
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Showing 4 changed files with 129 additions and 14 deletions.
diff --git a/expected/limit_offset_pushdown.out b/expected/limit_offset_pushdown.out
@@ -220,6 +220,58 @@ SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT 5 OFFSET NULL;
  50 | IT             | PUNE
 (5 rows)
 
+-- Limit with placeholder.
+EXPLAIN (VERBOSE, COSTS FALSE)
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (SELECT COUNT(*) FROM f_test_tbl2);
+                                                      QUERY PLAN                                                      
+----------------------------------------------------------------------------------------------------------------------
+ Foreign Scan on public.f_test_tbl2
+   Output: f_test_tbl2.c1, f_test_tbl2.c2, f_test_tbl2.c3
+   Remote query: SELECT `c1`, `c2`, `c3` FROM `mysql_fdw_regress`.`test_tbl2` ORDER BY `c1` IS NULL, `c1` ASC LIMIT ?
+   InitPlan 1 (returns $0)
+     ->  Foreign Scan
+           Output: (count(*))
+           Relations: Aggregate on (mysql_fdw_regress.f_test_tbl2)
+           Remote query: SELECT count(*) FROM `mysql_fdw_regress`.`test_tbl2`
+(8 rows)
+
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (SELECT COUNT(*) FROM f_test_tbl2);
+ c1 |       c2       |    c3    
+----+----------------+----------
+ 10 | DEVELOPMENT    | PUNE
+ 20 | ADMINISTRATION | BANGLORE
+ 30 | SALES          | MUMBAI
+ 40 | HR             | NAGPUR
+ 50 | IT             | PUNE
+ 60 | DB SERVER      | PUNE
+(6 rows)
+
+-- Limit with expression, should not pushdown.
+EXPLAIN (VERBOSE, COSTS FALSE)
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (10 - (SELECT COUNT(*) FROM f_test_tbl2));
+                                                     QUERY PLAN                                                     
+--------------------------------------------------------------------------------------------------------------------
+ Limit
+   Output: f_test_tbl2.c1, f_test_tbl2.c2, f_test_tbl2.c3
+   InitPlan 1 (returns $0)
+     ->  Foreign Scan
+           Output: (count(*))
+           Relations: Aggregate on (mysql_fdw_regress.f_test_tbl2)
+           Remote query: SELECT count(*) FROM `mysql_fdw_regress`.`test_tbl2`
+   ->  Foreign Scan on public.f_test_tbl2
+         Output: f_test_tbl2.c1, f_test_tbl2.c2, f_test_tbl2.c3
+         Remote query: SELECT `c1`, `c2`, `c3` FROM `mysql_fdw_regress`.`test_tbl2` ORDER BY `c1` IS NULL, `c1` ASC
+(10 rows)
+
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (10 - (SELECT COUNT(*) FROM f_test_tbl2));
+ c1 |       c2       |    c3    
+----+----------------+----------
+ 10 | DEVELOPMENT    | PUNE
+ 20 | ADMINISTRATION | BANGLORE
+ 30 | SALES          | MUMBAI
+ 40 | HR             | NAGPUR
+(4 rows)
+
 DELETE FROM f_test_tbl2;
 DROP FOREIGN TABLE f_test_tbl2;
 DROP USER MAPPING FOR public SERVER mysql_svr;

diff --git a/expected/limit_offset_pushdown_1.out b/expected/limit_offset_pushdown_1.out
@@ -230,6 +230,60 @@ SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT 5 OFFSET NULL;
  50 | IT             | PUNE
 (5 rows)
 
+-- Limit with placeholder.
+EXPLAIN (VERBOSE, COSTS FALSE)
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (SELECT COUNT(*) FROM f_test_tbl2);
+                                                     QUERY PLAN                                                     
+--------------------------------------------------------------------------------------------------------------------
+ Limit
+   Output: f_test_tbl2.c1, f_test_tbl2.c2, f_test_tbl2.c3
+   InitPlan 1 (returns $0)
+     ->  Foreign Scan
+           Output: (count(*))
+           Relations: Aggregate on (mysql_fdw_regress.f_test_tbl2)
+           Remote query: SELECT count(*) FROM `mysql_fdw_regress`.`test_tbl2`
+   ->  Foreign Scan on public.f_test_tbl2
+         Output: f_test_tbl2.c1, f_test_tbl2.c2, f_test_tbl2.c3
+         Remote query: SELECT `c1`, `c2`, `c3` FROM `mysql_fdw_regress`.`test_tbl2` ORDER BY `c1` IS NULL, `c1` ASC
+(10 rows)
+
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (SELECT COUNT(*) FROM f_test_tbl2);
+ c1 |       c2       |    c3    
+----+----------------+----------
+ 10 | DEVELOPMENT    | PUNE
+ 20 | ADMINISTRATION | BANGLORE
+ 30 | SALES          | MUMBAI
+ 40 | HR             | NAGPUR
+ 50 | IT             | PUNE
+ 60 | DB SERVER      | PUNE
+(6 rows)
+
+-- Limit with expression, should not pushdown.
+EXPLAIN (VERBOSE, COSTS FALSE)
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (10 - (SELECT COUNT(*) FROM f_test_tbl2));
+                                                     QUERY PLAN                                                     
+--------------------------------------------------------------------------------------------------------------------
+ Limit
+   Output: f_test_tbl2.c1, f_test_tbl2.c2, f_test_tbl2.c3
+   InitPlan 1 (returns $0)
+     ->  Foreign Scan
+           Output: (count(*))
+           Relations: Aggregate on (mysql_fdw_regress.f_test_tbl2)
+           Remote query: SELECT count(*) FROM `mysql_fdw_regress`.`test_tbl2`
+   ->  Foreign Scan on public.f_test_tbl2
+         Output: f_test_tbl2.c1, f_test_tbl2.c2, f_test_tbl2.c3
+         Remote query: SELECT `c1`, `c2`, `c3` FROM `mysql_fdw_regress`.`test_tbl2` ORDER BY `c1` IS NULL, `c1` ASC
+(10 rows)
+
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (10 - (SELECT COUNT(*) FROM f_test_tbl2));
+ c1 |       c2       |    c3    
+----+----------------+----------
+ 10 | DEVELOPMENT    | PUNE
+ 20 | ADMINISTRATION | BANGLORE
+ 30 | SALES          | MUMBAI
+ 40 | HR             | NAGPUR
+(4 rows)
+
 DELETE FROM f_test_tbl2;
 DROP FOREIGN TABLE f_test_tbl2;
 DROP USER MAPPING FOR public SERVER mysql_svr;

diff --git a/mysql_fdw.c b/mysql_fdw.c
@@ -4487,32 +4487,31 @@ mysql_add_foreign_final_paths(PlannerInfo *root, RelOptInfo *input_rel,
 		return;
 
 	/*
+	 * Support only Const and Param nodes as expressions are NOT suported.
 	 * MySQL doesn't support LIMIT/OFFSET NULL/ALL syntax, so check for the
 	 * same.  If const node is null then do not pushdown limit/offset clause.
 	 */
-	if (parse->limitCount && nodeTag(parse->limitCount) == T_Const)
+	if (parse->limitCount)
 	{
-		Const	   *c = (Const *) parse->limitCount;
+		if (nodeTag(parse->limitCount) != T_Const &&
+			nodeTag(parse->limitCount) != T_Param)
+			return;
 
-		if (c->constisnull)
+		if (nodeTag(parse->limitCount) == T_Const &&
+			((Const *) parse->limitCount)->constisnull)
 			return;
 	}
-	if (parse->limitOffset && nodeTag(parse->limitOffset) == T_Const)
+	if (parse->limitOffset)
 	{
-		Const	   *c = (Const *) parse->limitOffset;
+		if (nodeTag(parse->limitOffset) != T_Const &&
+			nodeTag(parse->limitOffset) != T_Param)
+			return;
 
-		if (c->constisnull)
+		if (nodeTag(parse->limitOffset) == T_Const &&
+			((Const *) parse->limitOffset)->constisnull)
 			return;
 	}
 
-	/*
-	 * Also, the LIMIT/OFFSET cannot be pushed down, if their expressions are
-	 * not safe to remote.
-	 */
-	if (!mysql_is_foreign_expr(root, input_rel, (Expr *) parse->limitOffset, true) ||
-		!mysql_is_foreign_expr(root, input_rel, (Expr *) parse->limitCount, true))
-		return;
-
 	/* Safe to push down */
 	fpinfo->pushdown_safe = true;
 

diff --git a/sql/limit_offset_pushdown.sql b/sql/limit_offset_pushdown.sql
@@ -76,6 +76,16 @@ EXPLAIN (VERBOSE, COSTS FALSE)
 SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT 5 OFFSET NULL;
 SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT 5 OFFSET NULL;
 
+-- Limit with placeholder.
+EXPLAIN (VERBOSE, COSTS FALSE)
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (SELECT COUNT(*) FROM f_test_tbl2);
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (SELECT COUNT(*) FROM f_test_tbl2);
+
+-- Limit with expression, should not pushdown.
+EXPLAIN (VERBOSE, COSTS FALSE)
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (10 - (SELECT COUNT(*) FROM f_test_tbl2));
+SELECT * FROM f_test_tbl2 ORDER BY 1 LIMIT (10 - (SELECT COUNT(*) FROM f_test_tbl2));
+
 DELETE FROM f_test_tbl2;
 DROP FOREIGN TABLE f_test_tbl2;
 DROP USER MAPPING FOR public SERVER mysql_svr;