子查询展开（subquery unnesting）

    2. Query OK, 0 rows affected (0.09 sec)
    4. obclient> create table t2(a int primary key, b int, c int);
    5. Query OK, 0 rows affected (0.09 sec)
    7. --- 有依赖关系的子查询被展开改写成连接
    8. obclient> explain select * from t1 where t1.a in (select t2.b from t2 where t2.c = t1.c);
    9. | ============================================
    11. |0 |MERGE JOIN      |     |9703     |215436|
    12. |1 | TABLE SCAN     |t1   |100000   |64066 |
    13. |2 | SORT           |     |10001    |129621|
    14. |3 |  SUBPLAN SCAN  |VIEW1|10001    |111242|
    15. |4 |   HASH DISTINCT|     |10001    |109862|
    16. |5 |    TABLE SCAN  |t2   |100000   |64066 |
    17. ============================================
    19. Outputs & filters:
    20.   0 - output([t1.a], [t1.b], [t1.c]), filter(nil),
    21.       equal_conds([t1.a = VIEW1.t2.b], [VIEW1.t2.c = t1.c]), other_conds(nil)
    22.   1 - output([t1.c], [t1.a], [t1.b]), filter(nil),
    23.       access([t1.c], [t1.a], [t1.b]), partitions(p0)
    24.   2 - output([VIEW1.t2.b], [VIEW1.t2.c]), filter(nil), sort_keys([VIEW1.t2.b, ASC], [VIEW1.t2.c, ASC])
    25.   3 - output([VIEW1.t2.b], [VIEW1.t2.c]), filter(nil),
    26.       access([VIEW1.t2.b], [VIEW1.t2.c])
    27.   4 - output([t2.b], [t2.c]), filter(nil),
    28.       distinct([t2.b], [t2.c])