Oracle入门教程：leading vs ordered hint

odered hint 可以指示oracle 使用from 关键字后面的表的顺序进行join连接！cbo会优先按照from 后面的表的顺序来进行join，当统计

leading hint 可以指示Oracle使用leading 中指定的表作为驱动表,
比如 正常的访问计划如下
SCOTT@> 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")

我们在leading 提示中指定 emp 表为驱动表
SCOTT@> 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")
如结果执行计划中将emp 作为驱动表！
1 在leading 提示同时使用ordered hint，则leading hint无效
SCOTT@> 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 使用两个冲突的leading hint ，则oracle cbo会忽略所有的leading 提示！
SCOTT@> 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")