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