如题，在测试SET_VAR时候，发现子查询中使用该Hint时，并不会生效。
mysql> SELECT /*+ SET_VAR(MAX_EXECUTION_TIME=1234) */ @@MAX_EXECUTION_TIME;
+----------------------+
| @@MAX_EXECUTION_TIME |
+----------------------+
| 1234 |
+----------------------+
1 row in set (0.01 sec)
mysql> 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 (0.00 sec)
mysql> SELECT a.inner_time from (select /*+ SET_VAR(MAX_EXECUTION_TIME=12345) */ @@MAX_EXECUTION_TIME as inner_time) a;
+------------+
| inner_time |
+------------+
| 600000 |
+------------+
1 row in set (0.00 sec)
