set_var not works for the subquery

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Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

SELECT /*+ SET_VAR(MAX_EXECUTION_TIME=1234) */ @@MAX_EXECUTION_TIME;
SELECT /*+ SET_VAR(MAX_EXECUTION_TIME=1234) */ @@MAX_EXECUTION_TIME, a.inner_time from (select /*+ SET_VAR(MAX_EXECUTION_TIME=12345) */ @@MAX_EXECUTION_TIME as inner_time) a;
SELECT a.inner_time from (select /*+ SET_VAR(MAX_EXECUTION_TIME=12345) */ @@MAX_EXECUTION_TIME as inner_time) a;

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

set_var affect the subquery

3. What did you see instead (Required)

set_var not works for the subquery

[14:55:54]TiDB root:test> SELECT /*+ SET_VAR(MAX_EXECUTION_TIME=1234) */ @@MAX_EXECUTION_TIME;
+----------------------+
| @@MAX_EXECUTION_TIME |
+----------------------+
| 1234                 |
+----------------------+
1 row in set
Time: 0.003s
[14:56:04]TiDB root:test> SELECT /*+ SET_VAR(MAX_EXECUTION_TIME=1234) */ @@MAX_EXECUTION_TIME, a.inner_time from (select /*+ SET_VAR(MAX_EXECUTION_TIME=12345) */ @@MAX_EXECUTION_TIME as inner_time) a;
+----------------------+------------+
| @@MAX_EXECUTION_TIME | inner_time |
+----------------------+------------+
| 1234                 | 1234       |
+----------------------+------------+
1 row in set
Time: 0.003s
[14:56:12]TiDB root:test> SELECT a.inner_time from (select /*+ SET_VAR(MAX_EXECUTION_TIME=12345) */ @@MAX_EXECUTION_TIME as inner_time) a;
+------------+
| inner_time |
+------------+
| 0          |
+------------+
1 row in set
Time: 0.004s

4. What is your TiDB version? (Required)

[14:58:09]TiDB root:test> select tidb_version();
+-----------------------------------------------------------+
| tidb_version()                                            |
+-----------------------------------------------------------+
| Release Version: v8.1.0                                   |
| Edition: Community                                        |
| Git Commit Hash: da39e1ca55c6051a1cbcd84dc230b8ba42f8075f |
| Git Branch: HEAD                                          |
| UTC Build Time: 2024-05-06 05:55:37                       |
| GoVersion: go1.21.6                                       |
| Race Enabled: false                                       |
| Check Table Before Drop: false                            |
| Store: tikv                                               |
+-----------------------------------------------------------+
1 row in set