MySQL关联查询优化实例
MySQL关联查询优化实例
项目中发现有一个查询响应非常慢,花时间分析以及优化,特地记录。(1)背景
项目采用MySQL数据库,操作使用Ibatis;
(2)查询说明
这个查询是每次查询一定数目的用户信息,查询中涉及到多表关联,具体查询SQL如下
- <select id="select" parameterClass="UserCompany" resultClass="UserCompany">
- SELECT S.NAME as name,
- S.IMAGE as image,
- S.ID as id,
- C.NAME as companyName,
- C.ID as companyId,
- A.FILE_NAME AS resourceFileName,
- A.FILE_PATH AS resourceFilePath,
- A.FILE_ID AS resourceFileId,
- COUNT(R.CONTACT) AS updResourceSize
- from sys_user_info S
- LEFT JOIN ATTACH_FILE_INFO A ON S.ID = A.USER_ID
- LEFT JOIN
- (SELECT RESOURCE_ID,CONTACT FROM company_resource_info
- WHERE UPDTIME >= #UPDResourceTime:TIMESTAMP# ) R
- ON S.ID = R.CONTACT,
- company_info C
- WHERE S.COMPANY_ID = C.ID
- GROUP BY S.ID
- LIMIT 15
- </select>
(3)分析过程
使用MySQL自带的profiler分析结果如下图
观察可得,99%的时间花在了拷贝数据到临时表上,也就是与其中的子查询有关系。
想来奇怪,如果只是LIMIT 15条数据,临时数据不应该花费这么多时间;个人觉得问题应该出在LIMIT对于里面的子查询无效,也就是里面的子查询会涉及到全部数据,从而导致临时表消耗很大的时间,这样就不难理解了。
(4)优化
想清楚了上面的原因,解决的思路也就比较清楚了,只要让里面涉及的子查询只查LIMIT对应的数据就可以了。
重新实行的方式如下:
- <resultMap class="cn.com.steel.wuyou.model.UserCompany" id="UserCompanyMap">
- <result property="name" column="name" />
- <result property="image" column="image" />
- <result property="id" column="id" />
- <result property="companyName" column="companyName" />
- <result property="companyId" column="companyId" />
- <result property="resourceFileName" column="resourceFileName" />
- <result property="resourceFilePath" column="resourceFilePath" />
- <result property="resourceFileId" column="resourceFileId" />
- <result property="UPDResourceTime" column="UPDResourceTime" />
- <result property="updResourceSize" column="{CONTACT=id,udpResourceTime=UPDResourceTime}"
- select="steel_userCompany.selectUpdResourceSize" />
- </resultMap>
- <select id="selectUpdResourceSize" parameterClass="java.util.HashMap"
- resultClass="int">
- SELECT COUNT(1) FROM company_resource_info
- WHERE CONTACT = #CONTACT#
- and UPDTIME >= #UPDResourceTime:TIMESTAMP#
- </select>
- <select id="select" parameterClass="UserCompany" resultMap="UserCompanyMap">
- SELECT S.NAME as name,
- S.IMAGE as image,
- S.ID as id,
- C.NAME as companyName,
- C.ID as companyId,
- A.FILE_NAME AS resourceFileName,
- A.FILE_PATH AS resourceFilePath,
- A.FILE_ID AS resourceFileId,
- #UPDResourceTime:TIMESTAMP# as UPDResourceTime
- from sys_user_info S
- LEFT JOIN ATTACH_FILE_INFO A ON S.ID = A.USER_ID
- LEFT JOIN company_info C ON S.COMPANY_ID = C.ID
- LIMIT 15
- </select>
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