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@www.linuxidc.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")
我们在leading 提示中指定 emp 表为驱动表
SCOTT@www.linuxidc.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@www.linuxidc.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 使用两个冲突的leading hint ，则oracle cbo会忽略所有的leading 提示！
SCOTT@www.linuxidc.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")