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)
，