子查询展开(subquery unnesting)

    2. Query OK, 0 rows affected (0.70 sec)
    3. OceanBase (root@test)> create table t2(a int primary key, b int, c int);
    4. Query OK, 0 rows affected (0.92 sec)
    5. --- 有依赖关系的子查询被展开改写成连接
    6. OceanBase (root@test)> explain select * from t1 where t1.a in (select t2.b from t2 where t2.c = t1.c);
    8. ---------------------------------------
    9. |0 |HASH SEMI JOIN|    |1        |2924|
    10. |1 | TABLE SCAN   |t1  |1000     |455 |
    11. |2 | TABLE SCAN   |t2  |1000     |455 |
    12. =======================================
    14. -------------------------------------
    15.   0 - output([t1.a], [t1.b], [t1.c]), filter(nil),
    16.       equal_conds([t1.a = t2.b], [t2.c = t1.c]), other_conds(nil)
    17.   1 - output([t1.c], [t1.a], [t1.b]), filter(nil),
    18.       access([t1.c], [t1.a], [t1.b]), partitions(p0)
    19.   2 - output([t2.c], [t2.b]), filter(nil),