oracle Swap_join_inputs简析

此hint仅针对hash_join有效,强行交换驱动关系;目标表为原hash连接的被驱动表,适合的场景:
1.多表的hash内连接(大于2个以上)
2.多表的hash外连接(大于等于2个以上)

场景1:

--2个表的内连接
select /*+ leading(d) use_hash(e) swap_join_inputs(e)*/  * from dept d,emp e
 where d.deptno=e.deptno;

这里写图片描述
可以看到,首先d作为驱动表,e被驱动,然后强行反转e;
此情况,没必要用swap_join_inputs,用leading就可以指定了;

场景2

--2个表的外连接
select /*+ leading(e) use_hash(d) */  * from dept d,emp e
 where d.deptno=e.deptno(+);

这里写图片描述

这里看到,leading失效了,那么可以使用swap_join_inputs(e)强行反转;
select /*+ leading(e) use_hash(d) swap_join_inputs(e)*/  * from dept d,emp e
 where d.deptno=e.deptno(+);

这里写图片描述

场景3:
多个表的内连接下交换顺序

drop table t1;
drop table t2;
drop table t3;
 create table t1
 as
 select * from dba_objects where rownum<2;   

 create table t2
 as
 select * from dba_objects where rownum<12;

 create table t3
 as
 select * from dba_objects where rownum<22;

select  /*+ ordered use_hash(t3)*/
    t1.owner, t2.data_object_id
   from t2, t3, t1
  where t2.object_id = t3.object_id
    and t1.object_id = t3.object_id;
或者
select  /*+ leading(t2) use_hash(t3)*/
    t1.owner, t2.data_object_id
   from t2, t3, t1
  where t2.object_id = t3.object_id
    and t1.object_id = t3.object_id;

这里写图片描述
如果想反过来,让T1作为第一个驱动表,怎么实现?
–方式1:leading 不对
select /+ leading(t1) use_hash(t2 t3)/
t1.owner, t2.data_object_id
from t2, t3, t1
where t2.object_id = t3.object_id
and t1.object_id = t3.object_id;

这里t2,t3虽然都作为了被驱动,但是顺序不对;

–方式2swap_join_inputs
select /+ leading(t2) use_hash(t3) swap_join_inputs(t1)/
t1.owner, t2.data_object_id
from t2, t3, t1
where t2.object_id = t3.object_id
and t1.object_id = t3.object_id;
这里写图片描述
这就对了

oracle Swap_join_inputs简析

猜你喜欢