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")

，

本文原创发布php中文网，转载请注明出处，感谢您的尊重！