MySQL优化之in、exists、join

一、inner join 、 in 、exists

1     explain
2     select a.id
3     from application as a
4     where exists(
5       select 1
6       from dispatch_app_history as d
7       where d.bomber_id = 165 and d.application_id = a.id
8     );

 

   

1     explain
2     select a.id
3     from application as a
4     where a.id in (
5       select d.application_id
6       from dispatch_app_history as d
7       where d.bomber_id = 165
8     );

 

    

1     explain
2     select a.id
3     from application as a
4     inner join dispatch_app_history as d on d.application_id = a.id
5     where d.bomber_id = 165;

分析：子查询需要application_id来关联外部表application，因为需要application_id字段，所以MySQL认为无法先执行这个子查询，而对application表进行全表查询。

结论：子查询、join查询具体性能怎么样需要根据实际情况决定

 

 

二、not in、not exists

    1、执行速度  

1      explain
2      select *
3      from dispatch_app as d
4      where d.application_id not in(
5         select dh.application_id
6         from dispatch_app_history as dh
7      );                       

     

 

    

1      explain
2      select *
3      from dispatch_app as d
4      where not exists (
5         select 1
6         from dispatch_app_history as dh
7         where d.application_id = dh.application_id
8      )

   

  结论：不考虑其他情况，通常情况下not exists的执行效率要高于not in

 

    2、条件中含有null

1      select *
2      from bomber as b
3      where not exists(
4         select 1
5         from dispatch_app_history as d
6         where d.partner_id = b.partner_id
7      )
8      group by b.partner_id;

 

     

1      select *
2      from bomber as b
3      where b.partner_id not in(
4         select distinct d.partner_id
5         from dispatch_app_history as d
6      );

  

结论：对于not in，条件中有null值的时候会直接停止执行返回null结果。对于not exists，条件中有null时会清除null后执行。in、exists的执行与not existst相同