YashanDB 如何确认执行计划中的驱动表

在nest loop中，如何通过测试手段确认驱动表是上方的表还是下方的表。

通过hint中的leading测试：

SQL> EXPLAIN SELECT /*+leading(a)*/ * FROM area a, branches b WHERE a.area_no = b.area_no;

PLAN_DESCRIPTION
----------------------------------------------------------------
SQL hash value: 2466524393
Optimizer: ADOPT_C

+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
| Id | Operation type                 | Name                 | Owner      | Rows     | Cost(%CPU)  | Partition info                 |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
|  0 | SELECT STATEMENT               |                      |            |          |             |                                |
|* 1 |  HASH JOIN INNER               |                      |            |        10|       11( 0)|                                |
|  2 |   TABLE ACCESS FULL            | AREA                 | SALES      |         5|        1( 0)|                                |
|  3 |   TABLE ACCESS FULL            | BRANCHES             | SALES      |        12|        1( 0)|                                |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+

Operation Information (identified by operation id):
---------------------------------------------------

   1 - Predicate : access("A"."AREA_NO" = "B"."AREA_NO")

16 rows fetched.

SQL> EXPLAIN SELECT /*+leading(b)*/ * FROM area a, branches b WHERE a.area_no = b.area_no;

PLAN_DESCRIPTION
----------------------------------------------------------------
SQL hash value: 809577705
Optimizer: ADOPT_C

+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
| Id | Operation type                 | Name                 | Owner      | Rows     | Cost(%CPU)  | Partition info                 |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+
|  0 | SELECT STATEMENT               |                      |            |          |             |                                |
|  1 |  NESTED INDEX LOOPS INNER      |                      |            |        10|        1( 0)|                                |
|  2 |   TABLE ACCESS FULL            | BRANCHES             | SALES      |        12|        1( 0)|                                |
|  3 |   TABLE ACCESS BY INDEX ROWID  | AREA                 | SALES      |         1|        1( 0)|                                |
|* 4 |    INDEX UNIQUE SCAN           | SYS_C_27             | SALES      |         1|        1( 0)|                                |
+----+--------------------------------+----------------------+------------+----------+-------------+--------------------------------+

Operation Information (identified by operation id):
---------------------------------------------------

   4 - Predicate : access("A"."AREA_NO" = "B"."AREA_NO")

17 rows fetched.

当使用leading(a)时，a表在上方；使用leading(b)时，b表在上方。可以确认在YashanDB的执行计划中，两表连接时，驱动表显示在上方。