当我获得下面查询的执行计划时，即使我写右外部联接，它也会写左外部联接。
select * from employees e right outer join departments d
using(department_id);
这是输出：
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 122 | 10980 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN OUTER | | 122 | 10980 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPARTMENTS | 27 | 567 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | DEPT_ID_PK | 27 | | 1 (0)| 00:00:01 |
|* 4 | SORT JOIN | | 107 | 7383 | 4 (25)| 00:00:01 |
| 5 | TABLE ACCESS FULL | EMPLOYEES | 107 | 7383 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("D"."DEPARTMENT_ID"="E"."DEPARTMENT_ID"(+))
filter("D"."DEPARTMENT_ID"="E"."DEPARTMENT_ID"(+))
如果我使用左外部联接，则这次显示右外部联接。是什么原因呢？
select * from employees e left outer join departments d using(department_id);
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 107 | 9630 | 6 (0)| 00:00:01 |
|* 1 | HASH JOIN RIGHT OUTER| | 107 | 9630 | 6 (0)| 00:00:01 |
| 2 | TABLE ACCESS FULL | DEPARTMENTS | 27 | 567 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL | EMPLOYEES | 107 | 7383 | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("E"."DEPARTMENT_ID"="D"."DEPARTMENT_ID"(+))
谢谢。