planner: set_var cannot take effect in UNION ALL statements

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t (a int);
insert into t values (1);
insert into t values (1);

mysql> select /*+ set_var(max_execution_time=100) */ a, sleep(1) from t;
ERROR 3024 (HY000): Query execution was interrupted, maximum statement execution time exceeded

mysql> select /*+ set_var(max_execution_time=100) */ a, sleep(1) from t union all select a, 1 from t;
+------+----------+
| a    | sleep(1) |
+------+----------+
|    1 |        1 |
|    1 |        1 |
|    1 |        0 |
|    1 |        0 |
+------+----------+
4 rows in set (2.01 sec)

2. What did you expect to see? (Required)

The second query should return an error.

3. What did you see instead (Required)

The second query's execution time exceeds the limitation.

The reason is that UNION ALL's node is not considered when extracting statement hints:

4. What is your TiDB version? (Required)

Master

[qw4990]

PTAL @hawkingrei Please take a look at this, THX!