Fix error result union scan with apply (#19245) (#19298)

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>

executor/union_scan.go

@@ -172,6 +172,15 @@ func (us *UnionScanExec) Next(ctx context.Context, req *chunk.Chunk) error {
 	return nil
 }
 
+// Close implements the Executor Close interface.
+func (us *UnionScanExec) Close() error {
+	us.cursor4AddRows = 0
+	us.cursor4SnapshotRows = 0
+	us.addedRows = us.addedRows[:0]
+	us.snapshotRows = us.snapshotRows[:0]
+	return us.children[0].Close()
+}
+
 // getOneRow gets one result row from dirty table or child.
 func (us *UnionScanExec) getOneRow(ctx context.Context) ([]types.Datum, error) {
 	for {

executor/union_scan_test.go

@@ -280,3 +280,17 @@ func (s *testSuite4) TestForUpdateUntouchedIndex(c *C) {
 	tk.MustExec("commit")
 	tk.MustExec("admin check table t")
 }
+
+// See https://github.com/pingcap/tidb/issues/19136
+func (s *testSuite4) TestForApplyAndUnionScan(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+
+	tk.MustExec("create table t ( c_int int, c_str varchar(40), primary key(c_int, c_str) )")
+	tk.MustExec("begin")
+	tk.MustExec("insert into t values (1, 'amazing almeida'), (2, 'boring bardeen'), (3, 'busy wescoff')")
+	tk.MustQuery("select c_int, (select t1.c_int from t t1 where t1.c_int = 3 and t1.c_int > t.c_int order by t1.c_int limit 1) x from t").Check(testkit.Rows("1 3", "2 3", "3 <nil>"))
+	tk.MustExec("commit")
+	tk.MustQuery("select c_int, (select t1.c_int from t t1 where t1.c_int = 3 and t1.c_int > t.c_int order by t1.c_int limit 1) x from t").Check(testkit.Rows("1 3", "2 3", "3 <nil>"))
+}