一条经典SQL语句优化实例

大家好，又见面了，我是全栈君。

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

发布者：全栈程序员栈长，转载请注明出处：https://javaforall.cn/108652.html原文链接：https://javaforall.cn