Allow RETURNING in fast-path queries (#3352)

* Allow RETURNING in fast-path queries

Because there is no specific reason for that.

src/backend/distributed/planner/fast_path_router_planner.c

@@ -162,7 +162,6 @@ GeneratePlaceHolderPlannedStmt(Query *parse)
  *      and it should be ANDed with any other filters. Also, the distribution
  *      key should only exists once in the WHERE clause. So basically,
  *          SELECT ... FROM dist_table WHERE dist_key = X
- *   - No returning for UPDATE/DELETE queries
  *   - All INSERT statements (including multi-row INSERTs) as long as the commands
  *     don't have any sublinks/CTEs etc
  */
@@ -181,9 +180,9 @@ FastPathRouterQuery(Query *query)
 	 * We want to deal with only very simple queries. Some of the
 	 * checks might be too restrictive, still we prefer this way.
 	 */
-	if (query->cteList != NIL || query->returningList != NIL ||
+	if (query->cteList != NIL || query->hasSubLinks ||
-		query->hasSubLinks || query->setOperations != NULL ||
+		query->setOperations != NULL || query->hasTargetSRFs ||
-		query->hasTargetSRFs || query->hasModifyingCTE)
+		query->hasModifyingCTE)
 	{
 		return false;
 	}

src/test/regress/expected/fast_path_router_modify.out

@@ -105,26 +105,55 @@ ERROR:  modifying the partition value of rows is not allowed
 UPDATE modify_fast_path SET key = 2::numeric WHERE key = 1;
 DEBUG:  modifying the partition value of rows is not allowed
 ERROR:  modifying the partition value of rows is not allowed
--- returning is not supported via fast-path
+-- returning is supported via fast-path
+INSERT INTO modify_fast_path (key, value_1) VALUES (1,1);
+DEBUG:  Creating router plan
+DEBUG:  Plan is router executable
+DETAIL:  distribution column value: 1
 DELETE FROM modify_fast_path WHERE key = 1 RETURNING *;
+DEBUG:  Distributed planning for a fast-path router query
 DEBUG:  Creating router plan
 DEBUG:  Plan is router executable
 DETAIL:  distribution column value: 1
  key | value_1 | value_2 
 -----+---------+---------
-(0 rows)
+   1 |       1 | 
+(1 row)
 
+INSERT INTO modify_fast_path (key, value_1) VALUES (2,1) RETURNING value_1, key;
+DEBUG:  Creating router plan
+DEBUG:  Plan is router executable
+DETAIL:  distribution column value: 2
+ value_1 | key 
+---------+-----
+       1 |   2
+(1 row)
+
+DELETE FROM modify_fast_path WHERE key = 2 RETURNING value_1 * 15, value_1::numeric * 16;
+DEBUG:  Distributed planning for a fast-path router query
+DEBUG:  Creating router plan
+DEBUG:  Plan is router executable
+DETAIL:  distribution column value: 2
+ ?column? | ?column? 
+----------+----------
+       15 |       16
+(1 row)
+
+-- still, non-immutable functions are not supported
+INSERT INTO modify_fast_path (key, value_1) VALUES (2,1) RETURNING value_1, random() * key;
+DEBUG:  non-IMMUTABLE functions are not allowed in the RETURNING clause
+ERROR:  non-IMMUTABLE functions are not allowed in the RETURNING clause
 -- modifying ctes are not supported via fast-path
 WITH t1 AS (DELETE FROM modify_fast_path WHERE key = 1), t2 AS  (SELECT * FROM modify_fast_path) SELECT * FROM t2;
 DEBUG:  data-modifying statements are not supported in the WITH clauses of distributed queries
-DEBUG:  generating subplan 18_1 for CTE t1: DELETE FROM fast_path_router_modify.modify_fast_path WHERE (key OPERATOR(pg_catalog.=) 1)
+DEBUG:  generating subplan 22_1 for CTE t1: DELETE FROM fast_path_router_modify.modify_fast_path WHERE (key OPERATOR(pg_catalog.=) 1)
 DEBUG:  Distributed planning for a fast-path router query
 DEBUG:  Creating router plan
 DEBUG:  Plan is router executable
 DETAIL:  distribution column value: 1
-DEBUG:  generating subplan 18_2 for CTE t2: SELECT key, value_1, value_2 FROM fast_path_router_modify.modify_fast_path
+DEBUG:  generating subplan 22_2 for CTE t2: SELECT key, value_1, value_2 FROM fast_path_router_modify.modify_fast_path
 DEBUG:  Router planner cannot handle multi-shard select queries
-DEBUG:  Plan 18 query after replacing subqueries and CTEs: SELECT key, value_1, value_2 FROM (SELECT intermediate_result.key, intermediate_result.value_1, intermediate_result.value_2 FROM read_intermediate_result('18_2'::text, 'binary'::citus_copy_format) intermediate_result(key integer, value_1 integer, value_2 text)) t2
+DEBUG:  Plan 22 query after replacing subqueries and CTEs: SELECT key, value_1, value_2 FROM (SELECT intermediate_result.key, intermediate_result.value_1, intermediate_result.value_2 FROM read_intermediate_result('22_2'::text, 'binary'::citus_copy_format) intermediate_result(key integer, value_1 integer, value_2 text)) t2
 DEBUG:  Creating router plan
 DEBUG:  Plan is router executable
  key | value_1 | value_2 

src/test/regress/sql/fast_path_router_modify.sql

@@ -53,8 +53,14 @@ UPDATE modify_fast_path SET key = 1::float WHERE key = 1;
 UPDATE modify_fast_path SET key = 2 WHERE key = 1;
 UPDATE modify_fast_path SET key = 2::numeric WHERE key = 1;
 
--- returning is not supported via fast-path
+-- returning is supported via fast-path
+INSERT INTO modify_fast_path (key, value_1) VALUES (1,1);
 DELETE FROM modify_fast_path WHERE key = 1 RETURNING *;
+INSERT INTO modify_fast_path (key, value_1) VALUES (2,1) RETURNING value_1, key;
+DELETE FROM modify_fast_path WHERE key = 2 RETURNING value_1 * 15, value_1::numeric * 16;
+
+-- still, non-immutable functions are not supported
+INSERT INTO modify_fast_path (key, value_1) VALUES (2,1) RETURNING value_1, random() * key;
 
 -- modifying ctes are not supported via fast-path
 WITH t1 AS (DELETE FROM modify_fast_path WHERE key = 1), t2 AS  (SELECT * FROM modify_fast_path) SELECT * FROM t2;