[release-21.0] bugfix: Plan group by only on top of derived tables correctly (#18505)

go/test/endtoend/vtgate/vitess_tester/aggregation/aggregation.test

@@ -11,6 +11,7 @@ CREATE TABLE `t2`
 (
     `id`    bigint unsigned NOT NULL AUTO_INCREMENT,
     `t1_id` int unsigned NOT NULL,
+    `col`   int unsigned,
     PRIMARY KEY (`id`)
 ) ENGINE InnoDB,
   CHARSET utf8mb4,
@@ -31,10 +32,10 @@ values (1, 'A'),
        (3, 'C'),
        (4, 'D');
 
-insert into t2 (id, t1_id)
-values (1, 1),
-       (2, 2),
-       (3, 3);
+insert into t2 (id, t1_id, col)
+values (1, 1, 1),
+       (2, 2, 2),
+       (3, 3, 3);
 
 insert into t3 (id, name)
 values (1, 'A'),
@@ -64,4 +65,10 @@ from (select id, count(*) as num_segments from t1 group by 1 order by 2 desc lim
          join t2 u on u.id = t.id;
 
 select name
-from (select name from t1 group by name having count(t1.id) > 1) t1;
+from (select name from t1 group by name having count(t1.id) > 1) t1;
+
+select t1_id
+from (select t1_id, col
+      from t2
+      group by 1, 2) t
+group by 1;

go/vt/vtgate/engine/ordered_aggregate.go

@@ -103,6 +103,7 @@ func (oa *OrderedAggregate) TryExecute(ctx context.Context, vcursor VCursor, bin
 	return qr.Truncate(oa.TruncateColumnCount), nil
 }
 
+// executeGroupBy is used when the plan contains grouping but not aggregations
 func (oa *OrderedAggregate) executeGroupBy(result *sqltypes.Result) (*sqltypes.Result, error) {
 	if len(result.Rows) < 1 {
 		return result, nil

go/vt/vtgate/planbuilder/operators/aggregator.go

@@ -292,7 +292,7 @@ func isDerived(op Operator) bool {
 }
 
 func (a *Aggregator) GetColumns(ctx *plancontext.PlanningContext) (res []*sqlparser.AliasedExpr) {
-	if isDerived(a.Source) {
+	if isDerived(a.Source) && len(a.Aggregations) > 0 {
 		return truncate(a, a.Columns)
 	}
 

go/vt/vtgate/planbuilder/testdata/aggr_cases.json

@@ -7406,5 +7406,43 @@
         "user.user_extra"
       ]
     }
+  },
+  {
+    "comment": "Group-only Aggregation on top of a derived table should return the correct number of columns",
+    "query": "select col from (select col, intcol from user group by 1, 2) t group by 1",
+    "plan": {
+      "QueryType": "SELECT",
+      "Original": "select col from (select col, intcol from user group by 1, 2) t group by 1",
+      "Instructions": {
+        "OperatorType": "Aggregate",
+        "Variant": "Ordered",
+        "GroupBy": "0",
+        "ResultColumns": 1,
+        "Inputs": [
+          {
+            "OperatorType": "Aggregate",
+            "Variant": "Ordered",
+            "GroupBy": "0, 1",
+            "Inputs": [
+              {
+                "OperatorType": "Route",
+                "Variant": "Scatter",
+                "Keyspace": {
+                  "Name": "user",
+                  "Sharded": true
+                },
+                "FieldQuery": "select col, intcol from `user` where 1 != 1 group by col, intcol",
+                "OrderBy": "0 ASC, 1 ASC",
+                "Query": "select col, intcol from `user` group by col, intcol order by col asc, intcol asc",
+                "Table": "`user`"
+              }
+            ]
+          }
+        ]
+      },
+      "TablesUsed": [
+        "user.user"
+      ]
+    }
   }
 ]