一条经典SQL语句优化实例

 1、概述

如下SQL语句发生严重消耗资源的问题，使得OS's load average会在30以上，一条语句需要执行上百秒。

/*
PIXPatient 184176条
DomainPatient 184189条
PersonName 184189条
*/

捕获的SQL语句：

select *

from PIXPatient where PIXPatientTID
in (select distinct PIXPatientTID from DomainPatient where DomainPatientTID
in ( select DomainPatientTID from DomainPatient
  where PatientBirthday = '1994-01-09' or PatientBirthday = '1994-01-01'
  union select  DomainPatientTID from PersonName where FamilyName = '倪' or GivenName = '界'));

2、优化

a.优化前执行效率：
mysql>

select * from PIXPatient where PIXPatientTID
in (select distinct PIXPatientTID from DomainPatient where DomainPatientTID
in ( select DomainPatientTID from DomainPatient
  where PatientBirthday = '1994-01-09' or PatientBirthday = '1994-01-01'
  union select  DomainPatientTID from PersonName where FamilyName = '倪' or GivenName = '界'));

b.加索引

alter table PersonName add index Index_FamilyName (FamilyName), add index Index_GivenName (GivenName);
alter table DomainPatient add index Index_PatientBirthday (PatientBirthday);

效果不明显

c.重构SQL语句（优化）
mysql>

select * from PIXPatient inner join (
    -> select distinct PIXPatientTID from DomainPatient inner join (
    -> select DomainPatientTID from DomainPatient  where PatientBirthday = '1994-01-09'
    -> union select DomainPatientTID from DomainPatient  where PatientBirthday = '1994-01-01'
    -> union select  DomainPatientTID from PersonName where FamilyName = '倪'
    -> union select  DomainPatientTID from PersonName where GivenName = '界' ) a using(DomainPatientTID) ) b using(PIXPatientTID) ;

效果明显

3、结论
SQL语句中，尽量避免使用or,in关键字，因为执行效率低。

规律：
join > exists > in
union > or