一条经典的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'));
+---------------+--------------+------------------+
| PIXPatientTID
| PIXPatientID | PIXPatientStatus
|
+---------------+--------------+------------------+
| 17013 |
17013 | active |
| 35491 | 35491 |
active |
| 39654 | 39654 | active |
| 47351 | 47351 | active |
| 77820 |
77820 | active |
| 111464 | 111464 |
active |
| 151700 | 151700 | active |
| 168993 | 168993 | active |
+---------------+--------------+------------------+
8 rows in set (1 min
58.11 sec)
8 rows in set (23.80 sec) ---- 空闲时
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)
;
+---------------+--------------+------------------+
| PIXPatientTID |
PIXPatientID | PIXPatientStatus
|
+---------------+--------------+------------------+
| 77820 |
77820 | active |
| 168993 | 168993 |
active |
| 17013 | 17013 | active |
| 35491 | 35491 | active |
| 39654 |
39654 | active |
| 47351 | 47351 |
active |
| 111464 | 111464 | active |
| 151700 | 151700 | active |
+---------------+--------------+------------------+
8 rows in set (1.11
sec)
8 rows in set (0.02 sec) ---- 空闲时
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