Improve error messages for INSERT queries that have subqueries

src/backend/distributed/planner/multi_router_planner.c

@@ -562,27 +562,15 @@ ModifyQuerySupported(Query *queryTree, Query *originalQuery, bool multiShardQuer
 	 */
 	if (queryTree->hasSubLinks == true)
 	{
-		/*
-		 * We support UPDATE and DELETE with subqueries unless they are multi
-		 * shard queries.
-		 */
+		/* we support subqueries for INSERTs only via INSERT INTO ... SELECT */
 		if (!UpdateOrDeleteQuery(queryTree))
 		{
-			StringInfo errorHint = makeStringInfo();
-			DistTableCacheEntry *cacheEntry = DistributedTableCacheEntry(
-				distributedTableId);
-			char *partitionKeyString = cacheEntry->partitionKeyString;
-			char *partitionColumnName = ColumnNameToColumn(distributedTableId,
-														   partitionKeyString);
-
-			appendStringInfo(errorHint,
-							 "Consider using an equality filter on partition column \"%s\" to target a single shard.",
-							 partitionColumnName);
+			Assert(queryTree->commandType == CMD_INSERT);
 
 			return DeferredError(ERRCODE_FEATURE_NOT_SUPPORTED,
-								 "subqueries are not supported in modifications across "
-								 "multiple shards",
-								 errorHint->data, NULL);
+								 "subqueries are not supported within INSERT queries",
+								 NULL, "Try rewriting your queries with 'INSERT "
+									   "INTO ... SELECT' syntax.");
 		}
 	}
 

src/test/regress/expected/multi_modifications.out

@@ -1275,6 +1275,20 @@ SELECT * FROM summary_table ORDER BY id;
 ----+-----------+---------------+-------+---------
 (0 rows)
 
+-- we don't support subqueries in VALUES clause
+INSERT INTO summary_table (id) VALUES ((SELECT id FROM summary_table));
+ERROR:  subqueries are not supported within INSERT queries
+HINT:  Try rewriting your queries with 'INSERT INTO ... SELECT' syntax.
+INSERT INTO summary_table (id) VALUES (5), ((SELECT id FROM summary_table));
+ERROR:  subqueries are not supported within INSERT queries
+HINT:  Try rewriting your queries with 'INSERT INTO ... SELECT' syntax.
+-- similar queries with reference tables
+INSERT INTO reference_summary_table (id) VALUES ((SELECT id FROM summary_table));
+ERROR:  subqueries are not supported within INSERT queries
+HINT:  Try rewriting your queries with 'INSERT INTO ... SELECT' syntax.
+INSERT INTO summary_table (id) VALUES ((SELECT id FROM reference_summary_table));
+ERROR:  subqueries are not supported within INSERT queries
+HINT:  Try rewriting your queries with 'INSERT INTO ... SELECT' syntax.
 DROP TABLE raw_table;
 DROP TABLE summary_table;
 DROP TABLE reference_raw_table;

src/test/regress/expected/multi_upsert.out

@@ -253,8 +253,8 @@ INSERT INTO dropcol_distributed AS dropcol (key, keep1) VALUES (1, '5') ON CONFL
 -- subquery in the SET clause
 INSERT INTO upsert_test (part_key, other_col) VALUES (1, 1) ON CONFLICT (part_key) DO
 	UPDATE SET other_col = (SELECT count(*) from upsert_test);
-ERROR:  subqueries are not supported in modifications across multiple shards
-DETAIL:  Consider using an equality filter on partition column "part_key" to target a single shard.
+ERROR:  subqueries are not supported within INSERT queries
+HINT:  Try rewriting your queries with 'INSERT INTO ... SELECT' syntax.
 -- non mutable function call in the SET
 INSERT INTO upsert_test (part_key, other_col) VALUES (1, 1) ON CONFLICT (part_key) DO
 	UPDATE SET other_col = random()::int;

src/test/regress/sql/multi_modifications.sql

@@ -821,6 +821,14 @@ EXECUTE prepared_delete_with_join(6);
 
 SELECT * FROM summary_table ORDER BY id;
 
+-- we don't support subqueries in VALUES clause
+INSERT INTO summary_table (id) VALUES ((SELECT id FROM summary_table));
+INSERT INTO summary_table (id) VALUES (5), ((SELECT id FROM summary_table));
+
+-- similar queries with reference tables
+INSERT INTO reference_summary_table (id) VALUES ((SELECT id FROM summary_table));
+INSERT INTO summary_table (id) VALUES ((SELECT id FROM reference_summary_table));
+
 DROP TABLE raw_table;
 DROP TABLE summary_table;
 DROP TABLE reference_raw_table;