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HomeDatabaseMysql TutorialIN&EXISTS与NOT IN&NOT EXISTS 的优化原则小结

下面只是从理论上提出了一些建议,最好的原则是大家在上面的基础上,能够使用执行计划来分析,得出最佳的语句的写法。

1. EXISTS的执行流程
select * from t1 where exists ( select null from t2 where y = x )
可以理解为:
代码如下:
for x in ( select * from t1 )
loop
if ( exists ( select null from t2 where y = x.x )
then
OUTPUT THE RECORD
end if
end loop

对于in 和 exists的性能区别:
如果子查询得出的结果集记录较少,主查询中的表较大且又有索引时应该用in,反之如果外层的主查询记录较少,子查询中的表大,又有索引时使用exists。
其实我们区分in和exists主要是造成了驱动顺序的改变(这是性能变化的关键),如果是exists,那么以外层表为驱动表,先被访问,如果是IN,那么先执行子查询,所以我们会以驱动表的快速返回为目标,那么就会考虑到索引及结果集的关系了
另外IN时不对NULL进行处理,如:
select 1 from dual where null in (0,1,2,null)
结果为空。

2. NOT IN 与NOT EXISTS:
NOT EXISTS的执行流程
代码如下:
select .....
from rollup R
where not exists ( select 'Found' from title T
where R.source_id = T.Title_ID);

可以理解为:
代码如下:
for x in ( select * from rollup )
loop
if ( not exists ( that query ) ) then
OUTPUT
end if;
end;

注意:NOT EXISTS 与 NOT IN 不能完全互相替换,看具体的需求。如果选择的列可以为空,则不能被替换。
例如下面语句,看他们的区别:
select x,y from t;
x y
------ ------
1 3
3 1
1 2
1 1
3 1
5
select * from t where x not in (select y from t t2 )
no rows
select * from t where not exists (select null from t t2
where t2.y=t.x )
x y
------ ------
5 NULL
所以要具体需求来决定
对于not in 和 not exists的性能区别:
not in 只有当子查询中,select 关键字后的字段有not null约束或者有这种暗示时用not in,另外如果主查询中表大,子查询中的表小但是记录多,则应当使用not in,并使用anti hash join.
如果主查询表中记录少,子查询表中记录多,并有索引,可以使用not exists,另外not in最好也可以用/*+ HASH_AJ */或者外连接+is null
NOT IN 在基于成本的应用中较好
比如:
代码如下:
select .....
from rollup R
where not exists ( select 'Found' from title T
where R.source_id = T.Title_ID);

改成(佳)
select ......
from title T, rollup R
where R.source_id = T.Title_id(+)
and T.Title_id is null;
或者(佳)
sql> select /*+ HASH_AJ */ ...
from rollup R
where ource_id NOT IN ( select ource_id
from title T
where ource_id IS NOT NULL )
注意:上面只是从理论上提出了一些建议,最好的原则是大家在上面的基础上,能够使用执行计划来分析,得出最佳的语句的写法。
'//=============================
exists,not exists总结

1 exists
SELECT * FROM anken_m WHERE EXISTS(
SELECT my_list_temp_m.sales_code
FROM my_list_temp_m
WHERE my_list_temp_m.sales_code=anken_m.sales_code)
说明:
1) 查询在anken_m表和my_list_temp_m表中都存在的sales_code。
2) sales_code是anken_m的主键,my_list_temp_m的外键。
注意:
1) 外层查询表anken_m是查询的对象。
2) 内层查询表my_list_temp_m是条件对象。
3) 内外层的查询表不能相同。
4) 作为关联条件的anken_m表不需要在内层查询FROM后添加。
5) my_list_temp_m.sales_code=anken_m.sales_code条件的左右顺序不影响查询结果。

2 not exists
SELECT * FROM anken_m WHERE NOT EXISTS(
SELECT my_list_temp_m.sales_code
FROM my_list_temp_m
WHERE my_list_temp_m.sales_code=anken_m.sales_code)
说明:
1) 查询在anken_m表中存在,但是在my_list_temp_m表中不存在的sales_code。
2) sales_code是anken_m的主键,my_list_temp_m的外键。
注意:
1) 外层查询表anken_m是查询的对象。
2) 内层查询表my_list_temp_m是条件对象。
3) 内外层的查询表不能相同。
4) 作为关联条件的anken_m表不需要在内层查询FROM后添加。
5) my_list_temp_m.sales_code=anken_m.sales_code条件的左右顺序不影响查询结果。

3 综合运用
UPDATE anken_m
SET(plan_type_code, branch_name, business_type_code)
=(SELECT anken.plan_type_code,anken.branch_name,anken.business_type_code
FROM anken
WHERE anken.sales_code=anken_m.sales_code)
WHERE EXISTS (
SELECT anken.sales_code
FROM anken,my_list_temp_m
WHERE my_list_temp_m.sales_code=anken.sales_code
AND anken.sales_code=anken_m.sales_code
)
说明:
1) 用一个表的记录数据更新另一个表的记录数据。
2) 用一个SQL语句进行批量更新。
2) sales_code是anken,anken_m的主键,my_list_temp_m的外键。
注意:
1) set 语句中的要被更新字段必须跟数据源字段一一对应,另外数据源查询中的条件必须限定一条记录。也就是根据sales_code可以唯一确定anken的一条记录,和anken_m的一条记录,这样才能保证要被更新的记录和数据源记录的主键是相同的。
2) 根据WHERE EXISTS语句可以确定数据源记录的范围,也就是可以用anken表中哪些记录更新anken_m表。所以anken_m不需要在WHERE EXISTS语句中的FROM后添加。
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