Oracle入門教程：leading vs ordered hint

leading hint 可以指示Oracle使用leading 中指定的表作為驅動表,
比如 正常的訪問計劃如下
SCOTT@www.bkjia.com> select e.ename, hiredate, b.comm
  2   from emp e, bonus b
  3   where e.ename = b.ename;
Execution Plan
----------------------------------------------------------
Plan hash value: 1125985041
----------------------------------------------------------------------------
| Id  | Operation          | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |       |     1 |    34 |     6  (17)| 00:00:01 |
|*  1 |  HASH JOIN         |       |     1 |    34 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| BONUS |     1 |    20 |     2   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| EMP   |    14 |   196 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("E"."ENAME"="B"."ENAME")

我們在leading 提示中指定 emp 表為驅動表
SCOTT@www.bkjia.com> select /*+ leading(e b) */ e.ename,hiredate,b.comm

  2   from emp e, bonus b
  3   where e.ename = b.ename;
Execution Plan
----------------------------------------------------------
Plan hash value: 1842254584
----------------------------------------------------------------------------
| Id  | Operation          | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |       |     1 |    34 |     6  (17)| 00:00:01 |
|*  1 |  HASH JOIN         |       |     1 |    34 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| EMP   |    14 |   196 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| BONUS |     1 |    20 |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("E"."ENAME"="B"."ENAME")
如結果執行計畫中將emp 作為驅動表！
1 在leading 提示同時使用ordered hint，則leading hint無效
SCOTT@www.bkjia.com> select /*+ leading(b e)  ordered */  e.ename,hiredate, b.comm

  2   from emp e, bonus b
  3   where e.ename = b.ename;
Execution Plan
----------------------------------------------------------
Plan hash value: 1842254584
----------------------------------------------------------------------------
| Id  | Operation          | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |       |     1 |    34 |     6  (17)| 00:00:01 |
|*  1 |  HASH JOIN         |       |     1 |    34 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| EMP   |    14 |   196 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| BONUS |     1 |    20 |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("E"."ENAME"="B"."ENAME")
2 使用兩個衝突的leading hint ，則oracle cbo會忽略所有的leading 提示！
SCOTT@www.bkjia.com> select /*+ leading(b e) leading(e b) */e.ename,hiredate, b.comm

  2   from emp e, bonus b
  3   where e.ename = b.ename;
Execution Plan
----------------------------------------------------------
Plan hash value: 1125985041
----------------------------------------------------------------------------
| Id  | Operation          | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |       |     1 |    34 |     6  (17)| 00:00:01 |
|*  1 |  HASH JOIN         |       |     1 |    34 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| BONUS |     1 |    20 |     2   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| EMP   |    14 |   196 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("E"."ENAME"="B"."ENAME")

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