Require subquery_pushdown when limit is used in subquery

Using limit in subqueries may cause returning incorrect
results. Therefore we allow limits in subqueries only
if user explicitly set subquery_pushdown flag.

src/backend/distributed/planner/multi_logical_planner.c

@@ -71,9 +71,11 @@ static DeferredErrorMessage * DeferErrorIfUnsupportedFilters(Query *subquery);
 static bool EqualOpExpressionLists(List *firstOpExpressionList,
 								   List *secondOpExpressionList);
 static DeferredErrorMessage * DeferErrorIfCannotPushdownSubquery(Query *subqueryTree,
-																 bool outerQueryHasLimit);
+																 bool
+																 outerMostQueryHasLimit);
 static DeferredErrorMessage * DeferErrorIfUnsupportedUnionQuery(Query *queryTree,
-																bool outerQueryHasLimit);
+																bool
+																outerMostQueryHasLimit);
 static bool ExtractSetOperationStatmentWalker(Node *node, List **setOperationList);
 static DeferredErrorMessage * DeferErrorIfUnsupportedTableCombination(Query *queryTree);
 static bool TargetListOnPartitionColumn(Query *query, List *targetEntryList);
@@ -335,14 +337,14 @@ DeferErrorIfUnsupportedSubqueryPushdown(Query *originalQuery,
 {
 	ListCell *rangeTableEntryCell = NULL;
 	List *subqueryEntryList = NIL;
-	bool outerQueryHasLimit = false;
+	bool outerMostQueryHasLimit = false;
 	DeferredErrorMessage *error = NULL;
 	RelationRestrictionContext *relationRestrictionContext =
 		plannerRestrictionContext->relationRestrictionContext;
 
 	if (originalQuery->limitCount != NULL)
 	{
-		outerQueryHasLimit = true;
+		outerMostQueryHasLimit = true;
 	}
 
 	/*
@@ -380,7 +382,7 @@ DeferErrorIfUnsupportedSubqueryPushdown(Query *originalQuery,
 		RangeTblEntry *rangeTableEntry = lfirst(rangeTableEntryCell);
 		Query *subquery = rangeTableEntry->subquery;
 
-		error = DeferErrorIfCannotPushdownSubquery(subquery, outerQueryHasLimit);
+		error = DeferErrorIfCannotPushdownSubquery(subquery, outerMostQueryHasLimit);
 		if (error)
 		{
 			return error;
@@ -533,7 +535,7 @@ EqualOpExpressionLists(List *firstOpExpressionList, List *secondOpExpressionList
  * features of underlying tables.
  */
 static DeferredErrorMessage *
-DeferErrorIfCannotPushdownSubquery(Query *subqueryTree, bool outerQueryHasLimit)
+DeferErrorIfCannotPushdownSubquery(Query *subqueryTree, bool outerMostQueryHasLimit)
 {
 	bool preconditionsSatisfied = true;
 	char *errorDetail = NULL;
@@ -571,16 +573,28 @@ DeferErrorIfCannotPushdownSubquery(Query *subqueryTree, bool outerQueryHasLimit)
 		errorDetail = "Offset clause is currently unsupported";
 	}
 
-	if (subqueryTree->limitCount && !outerQueryHasLimit)
+	/* limit is not supported when SubqueryPushdown is not set */
+	if (subqueryTree->limitCount && !SubqueryPushdown)
 	{
 		preconditionsSatisfied = false;
-		errorDetail = "Limit in subquery without limit in the outer query is unsupported";
+		errorDetail = "Limit in subquery is currently unsupported";
+	}
+
+	/*
+	 * Limit is partially supported when SubqueryPushdown is set.
+	 * The outermost query must have a limit clause.
+	 */
+	if (subqueryTree->limitCount && SubqueryPushdown && !outerMostQueryHasLimit)
+	{
+		preconditionsSatisfied = false;
+		errorDetail = "Limit in subquery without limit in the outermost query is "
+					  "unsupported";
 	}
 
 	if (subqueryTree->setOperations)
 	{
 		deferredError = DeferErrorIfUnsupportedUnionQuery(subqueryTree,
-														  outerQueryHasLimit);
+														  outerMostQueryHasLimit);
 		if (deferredError)
 		{
 			return deferredError;
@@ -671,7 +685,7 @@ DeferErrorIfCannotPushdownSubquery(Query *subqueryTree, bool outerQueryHasLimit)
 
 		Query *innerSubquery = rangeTableEntry->subquery;
 		deferredError = DeferErrorIfCannotPushdownSubquery(innerSubquery,
-														   outerQueryHasLimit);
+														   outerMostQueryHasLimit);
 		if (deferredError)
 		{
 			return deferredError;
@@ -690,7 +704,7 @@ DeferErrorIfCannotPushdownSubquery(Query *subqueryTree, bool outerQueryHasLimit)
  */
 static DeferredErrorMessage *
 DeferErrorIfUnsupportedUnionQuery(Query *subqueryTree,
-								  bool outerQueryHasLimit)
+								  bool outerMostQueryHasLimit)
 {
 	List *rangeTableIndexList = NIL;
 	ListCell *rangeTableIndexCell = NULL;
@@ -724,7 +738,7 @@ DeferErrorIfUnsupportedUnionQuery(Query *subqueryTree,
 		Assert(rangeTableEntry->rtekind == RTE_SUBQUERY);
 
 		deferredError = DeferErrorIfCannotPushdownSubquery(rangeTableEntry->subquery,
-														   outerQueryHasLimit);
+														   outerMostQueryHasLimit);
 		if (deferredError)
 		{
 			return deferredError;

src/test/regress/expected/multi_subquery_behavioral_analytics.out

@@ -830,6 +830,9 @@ GROUP BY
               1
 (1 row)
 
+-- most queries below has limit clause
+-- therefore setting subquery_pushdown flag for all
+SET citus.subquery_pushdown to ON;
 -- multi-subquery-join
 -- The first query has filters on partion column to make it router plannable
 -- but it is processed by logical planner since we disabled router execution
@@ -1947,4 +1950,5 @@ FROM (
 ERROR:  unsupported offset clause
 SET client_min_messages TO DEFAULT;
 DROP FUNCTION volatile_func_test();
+SET citus.subquery_pushdown to OFF;
 SET citus.enable_router_execution TO TRUE;

src/test/regress/expected/multi_subquery_complex_queries.out

@@ -1115,6 +1115,8 @@ LIMIT 10;
 (10 rows)
 
 -- Simple LATERAL JOINs with GROUP BYs in each side
+-- need to set subquery_pushdown due to limit for next 2 queries
+SET citus.subquery_pushdown to ON;
 SELECT *
 FROM
   (SELECT "some_users_data".user_id, lastseen
@@ -1205,6 +1207,8 @@ limit 50;
       34 | Tue Jan 21 04:15:03.874341 2014
 (6 rows)
 
+-- reset subquery_pushdown
+SET citus.subquery_pushdown to OFF;
 -- not supported since JOIN is not on the partition key
 SELECT "some_users_data".user_id, lastseen
 FROM
@@ -1277,6 +1281,7 @@ limit 50;
 ERROR:  cannot pushdown the subquery since all relations are not joined using distribution keys
 DETAIL:  Each relation should be joined with at least one another relation using distribution keys and equality operator.
 -- LATERAL JOINs used with INNER JOINs
+SET citus.subquery_pushdown to ON;
 SELECT user_id, lastseen
 FROM
   (SELECT 
@@ -1405,6 +1410,7 @@ LIMIT 10;
       14 | Tue Jan 21 05:46:51.286381 2014 |    10
 (3 rows)
 
+SET citus.subquery_pushdown to OFF;
 -- not supported since the inner JOIN is not equi join
 SELECT user_id, lastseen
 FROM
@@ -1866,6 +1872,7 @@ ORDER BY cnt, value_3 DESC LIMIT 10;
 ERROR:  cannot pushdown the subquery since all relations are not joined using distribution keys
 DETAIL:  Each relation should be joined with at least one another relation using distribution keys and equality operator.
 -- nested LATERAL JOINs
+SET citus.subquery_pushdown to ON;
 SELECT *
 FROM
   (SELECT "some_users_data".user_id, "some_recent_users".value_3
@@ -2070,6 +2077,7 @@ LIMIT 10;
       21 |     985
 (6 rows)
 
+SET citus.subquery_pushdown to OFF;
 -- LEFT JOINs used with INNER JOINs
 SELECT
 count(*) AS cnt, "generated_group_field"
@@ -2154,6 +2162,7 @@ LIMIT 10;
 (10 rows)
 
 -- lateral joins in the nested manner
+SET citus.subquery_pushdown to ON;
 SELECT *
 FROM
   (SELECT 
@@ -2204,6 +2213,7 @@ LIMIT 10;
       19 |     966
 (9 rows)
 
+SET citus.subquery_pushdown to OFF;
 -- not supported since join is not on the partition key
 SELECT *
 FROM

src/test/regress/input/multi_subquery.source

@@ -193,7 +193,7 @@ SELECT count(*) FROM
    (SELECT l_orderkey FROM lineitem_subquery) UNION
    (SELECT l_orderkey FROM lineitem_subquery)
 ) b;
--- Check that we error out if inner query has limit but outer quers has not.
+-- Check that we error out if inner query has Limit but subquery_pushdown is not set
 SELECT
 	avg(o_totalprice/l_quantity)
 FROM
@@ -214,6 +214,32 @@ FROM
 		WHERE
 			lineitem_quantities.l_orderkey = o_orderkey) orders_price ON true;
 
+-- Limit is only supported when subquery_pushdown is set
+-- Check that we error out if inner query has limit but outer query has not.
+SET citus.subquery_pushdown to ON;
+SELECT
+	avg(o_totalprice/l_quantity)
+FROM
+		(SELECT
+			l_orderkey,
+			l_quantity
+		FROM
+			lineitem_subquery
+		ORDER BY
+			l_quantity
+		LIMIT 10
+		) lineitem_quantities
+	JOIN LATERAL
+		(SELECT
+			o_totalprice
+		FROM
+			orders_subquery
+		WHERE
+			lineitem_quantities.l_orderkey = o_orderkey) orders_price ON true;
+
+-- reset the flag for next query
+SET citus.subquery_pushdown to OFF;
+
 -- Check that we error out if the outermost query is a distinct clause.
 
 SELECT
@@ -1083,6 +1109,8 @@ ORDER BY
 	count_pay;
 	
 -- Lateral join subquery pushdown
+-- set subquery_pushdown since there is limit in the query
+SET citus.subquery_pushdown to ON;
 SELECT
 	tenant_id,
 	user_id,
@@ -1132,7 +1160,8 @@ ORDER BY
 	user_lastseen DESC
 LIMIT
 	10;
-
+-- reset the flag for next query
+SET citus.subquery_pushdown to OFF;
 
 -- Same queries above with explain
 -- Simple join subquery pushdown
@@ -1297,6 +1326,8 @@ ORDER BY
 	count_pay;
 
 -- Lateral join subquery pushdown
+-- set subquery_pushdown due to limit in the query
+SET citus.subquery_pushdown to ON;
 EXPLAIN SELECT
 	tenant_id,
 	user_id,
@@ -1346,5 +1377,5 @@ ORDER BY
 	user_lastseen DESC
 LIMIT
 	10;
-	
+SET citus.subquery_pushdown to OFF;
 SET citus.enable_router_execution TO 'true';

src/test/regress/output/multi_subquery.source

@@ -201,7 +201,7 @@ SELECT count(*) FROM
   2985
 (1 row)
 
--- Check that we error out if inner query has limit but outer quers has not.
+-- Check that we error out if inner query has Limit but subquery_pushdown is not set
 SELECT
 	avg(o_totalprice/l_quantity)
 FROM
@@ -222,7 +222,33 @@ FROM
 		WHERE
 			lineitem_quantities.l_orderkey = o_orderkey) orders_price ON true;
 ERROR:  cannot push down this subquery
-DETAIL:  Limit in subquery without limit in the outer query is unsupported
+DETAIL:  Limit in subquery is currently unsupported
+-- Limit is only supported when subquery_pushdown is set
+-- Check that we error out if inner query has limit but outer query has not.
+SET citus.subquery_pushdown to ON;
+SELECT
+	avg(o_totalprice/l_quantity)
+FROM
+		(SELECT
+			l_orderkey,
+			l_quantity
+		FROM
+			lineitem_subquery
+		ORDER BY
+			l_quantity
+		LIMIT 10
+		) lineitem_quantities
+	JOIN LATERAL
+		(SELECT
+			o_totalprice
+		FROM
+			orders_subquery
+		WHERE
+			lineitem_quantities.l_orderkey = o_orderkey) orders_price ON true;
+ERROR:  cannot push down this subquery
+DETAIL:  Limit in subquery without limit in the outermost query is unsupported
+-- reset the flag for next query
+SET citus.subquery_pushdown to OFF;
 -- Check that we error out if the outermost query is a distinct clause.
 SELECT
 	count(DISTINCT a)
@@ -1084,6 +1110,8 @@ ORDER BY
 
 	
 -- Lateral join subquery pushdown
+-- set subquery_pushdown since there is limit in the query
+SET citus.subquery_pushdown to ON;
 SELECT
 	tenant_id,
 	user_id,
@@ -1140,6 +1168,8 @@ LIMIT
          1 |    1001 |    1472807115 | {click,submit,pay}
 (3 rows)
 
+-- reset the flag for next query
+SET citus.subquery_pushdown to OFF;
 -- Same queries above with explain
 -- Simple join subquery pushdown
 EXPLAIN SELECT
@@ -1367,6 +1397,8 @@ ORDER BY
 	count_pay;
 ERROR:  bogus varattno for OUTER_VAR var: 3
 -- Lateral join subquery pushdown
+-- set subquery_pushdown due to limit in the query
+SET citus.subquery_pushdown to ON;
 EXPLAIN SELECT
 	tenant_id,
 	user_id,
@@ -1447,5 +1479,5 @@ LIMIT
                                                                      Filter: (composite_id = users.composite_id)
 (27 rows)
 
-	
+SET citus.subquery_pushdown to OFF;
 SET citus.enable_router_execution TO 'true';

src/test/regress/output/multi_subquery_0.source

@@ -201,7 +201,7 @@ SELECT count(*) FROM
   2985
 (1 row)
 
--- Check that we error out if inner query has limit but outer quers has not.
+-- Check that we error out if inner query has Limit but subquery_pushdown is not set
 SELECT
 	avg(o_totalprice/l_quantity)
 FROM
@@ -222,7 +222,33 @@ FROM
 		WHERE
 			lineitem_quantities.l_orderkey = o_orderkey) orders_price ON true;
 ERROR:  cannot push down this subquery
-DETAIL:  Limit in subquery without limit in the outer query is unsupported
+DETAIL:  Limit in subquery is currently unsupported
+-- Limit is only supported when subquery_pushdown is set
+-- Check that we error out if inner query has limit but outer query has not.
+SET citus.subquery_pushdown to ON;
+SELECT
+	avg(o_totalprice/l_quantity)
+FROM
+		(SELECT
+			l_orderkey,
+			l_quantity
+		FROM
+			lineitem_subquery
+		ORDER BY
+			l_quantity
+		LIMIT 10
+		) lineitem_quantities
+	JOIN LATERAL
+		(SELECT
+			o_totalprice
+		FROM
+			orders_subquery
+		WHERE
+			lineitem_quantities.l_orderkey = o_orderkey) orders_price ON true;
+ERROR:  cannot push down this subquery
+DETAIL:  Limit in subquery without limit in the outermost query is unsupported
+-- reset the flag for next query
+SET citus.subquery_pushdown to OFF;
 -- Check that we error out if the outermost query is a distinct clause.
 SELECT
 	count(DISTINCT a)
@@ -1084,6 +1110,8 @@ ORDER BY
 
 	
 -- Lateral join subquery pushdown
+-- set subquery_pushdown since there is limit in the query
+SET citus.subquery_pushdown to ON;
 SELECT
 	tenant_id,
 	user_id,
@@ -1140,6 +1168,8 @@ LIMIT
          1 |    1001 |    1472807115 | {click,submit,pay}
 (3 rows)
 
+-- reset the flag for next query
+SET citus.subquery_pushdown to OFF;
 -- Same queries above with explain
 -- Simple join subquery pushdown
 EXPLAIN SELECT
@@ -1351,6 +1381,8 @@ ORDER BY
 	count_pay;
 ERROR:  bogus varattno for OUTER_VAR var: 3
 -- Lateral join subquery pushdown
+-- set subquery_pushdown due to limit in the query
+SET citus.subquery_pushdown to ON;
 EXPLAIN SELECT
 	tenant_id,
 	user_id,
@@ -1430,5 +1462,5 @@ LIMIT
                                                                      Filter: (composite_id = composite_id)
 (26 rows)
 
-	
+SET citus.subquery_pushdown to OFF;
 SET citus.enable_router_execution TO 'true';

src/test/regress/sql/multi_subquery_behavioral_analytics.sql

@@ -697,6 +697,10 @@ WHERE
 GROUP BY 
   a.user_id;
 
+-- most queries below has limit clause
+-- therefore setting subquery_pushdown flag for all
+SET citus.subquery_pushdown to ON;
+
 -- multi-subquery-join
 -- The first query has filters on partion column to make it router plannable
 -- but it is processed by logical planner since we disabled router execution
@@ -1596,4 +1600,5 @@ FROM (
 SET client_min_messages TO DEFAULT;
 DROP FUNCTION volatile_func_test();
 
+SET citus.subquery_pushdown to OFF;
 SET citus.enable_router_execution TO TRUE;

src/test/regress/sql/multi_subquery_complex_queries.sql

@@ -1043,6 +1043,8 @@ GROUP BY
 LIMIT 10;
 
 -- Simple LATERAL JOINs with GROUP BYs in each side
+-- need to set subquery_pushdown due to limit for next 2 queries
+SET citus.subquery_pushdown to ON;
 SELECT *
 FROM
   (SELECT "some_users_data".user_id, lastseen
@@ -1115,6 +1117,9 @@ ORDER BY
   user_id
 limit 50;
 
+-- reset subquery_pushdown
+SET citus.subquery_pushdown to OFF;
+
 -- not supported since JOIN is not on the partition key
 SELECT "some_users_data".user_id, lastseen
 FROM
@@ -1185,6 +1190,7 @@ ORDER BY
 limit 50;
 
 -- LATERAL JOINs used with INNER JOINs
+SET citus.subquery_pushdown to ON;
 SELECT user_id, lastseen
 FROM
   (SELECT 
@@ -1294,6 +1300,8 @@ GROUP BY 1
 ORDER BY 2, 1 DESC
 LIMIT 10;
 
+SET citus.subquery_pushdown to OFF;
+
 -- not supported since the inner JOIN is not equi join
 SELECT user_id, lastseen
 FROM
@@ -1718,6 +1726,7 @@ GROUP BY "value_3"
 ORDER BY cnt, value_3 DESC LIMIT 10;
 
 -- nested LATERAL JOINs
+SET citus.subquery_pushdown to ON;
 SELECT *
 FROM
   (SELECT "some_users_data".user_id, "some_recent_users".value_3
@@ -1886,6 +1895,8 @@ FROM
 ORDER BY value_3 DESC
 LIMIT 10;
 
+SET citus.subquery_pushdown to OFF;
+
 -- LEFT JOINs used with INNER JOINs
 SELECT
 count(*) AS cnt, "generated_group_field"
@@ -1944,6 +1955,7 @@ ORDER BY
 LIMIT 10;
 
 -- lateral joins in the nested manner
+SET citus.subquery_pushdown to ON;
 SELECT *
 FROM
   (SELECT 
@@ -1981,6 +1993,7 @@ FROM
 ORDER BY 
     value_2 DESC, user_id DESC
 LIMIT 10;
+SET citus.subquery_pushdown to OFF;
 
 -- not supported since join is not on the partition key
 SELECT *