executor: MaxOneRow operator should keep its promise

executor/executor.go

@@ -899,11 +899,17 @@ func (e *MaxOneRowExec) Next(ctx context.Context, chk *chunk.Chunk) error {
 			chk.AppendNull(i)
 		}
 		return nil
-	} else if num == 1 {
-		return nil
+	} else if num != 1 {
+		return errors.New("subquery returns more than 1 row")
+	}
+
+	childChunk := e.children[0].newChunk()
+	err = e.children[0].Next(ctx, childChunk)
+	if childChunk.NumRows() != 0 {
+		return errors.New("subquery returns more than 1 row")
 	}
 
-	return errors.New("subquery returns more than 1 row")
+	return nil
 }
 
 // UnionExec pulls all it's children's result and returns to its parent directly.

executor/executor_test.go

@@ -3068,3 +3068,23 @@ func (s *testSuite) TestUpdateJoin(c *C) {
 	tk.MustQuery("select k, v from t5").Check(testkit.Rows("0 0"))
 
 }
+
+func (s *testSuite) TestMaxOneRow(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec(`use test`)
+	tk.MustExec(`drop table if exists t1`)
+	tk.MustExec(`drop table if exists t2`)
+	tk.MustExec(`create table t1(a double, b double);`)
+	tk.MustExec(`create table t2(a double, b double);`)
+	tk.MustExec(`insert into t1 values(1, 1), (2, 2), (3, 3);`)
+	tk.MustExec(`insert into t2 values(0, 0);`)
+	tk.MustExec(`set @@tidb_max_chunk_size=1;`)
+	rs, err := tk.Exec(`select (select t1.a from t1 where t1.a > t2.a) as a from t2;`)
+	c.Assert(err, IsNil)
+
+	err = rs.Next(context.TODO(), rs.NewChunk())
+	c.Assert(err.Error(), Equals, "subquery returns more than 1 row")
+
+	err = rs.Close()
+	c.Assert(err, IsNil)
+}