[MySQL] MySQL联表查询的执行顺序优化查询

SELECT t4.orgName,
                   t3.projectName,
                   t3.Partner,
                   t1.type,
                   COUNT(DISTINCT t1.imei) AS count
            FROM `t_teminal` AS t1
            LEFT JOIN `t_orgcode_orgid_rela` AS t2
              ON t1.orgid = t2.orgId
            LEFT JOIN `t_org_code` AS t3
            ON t2.topOrganizationCode = t3.topOrganizationCode
            LEFT JOIN `t_organization` AS t4
            ON t1.orgid = t4.orgId
            GROUP BY t1.orgid

　　

联表查询会涉及到多个表的操作, 在索引的利用上会受限制. 上面的命令执行时间720s

以上面为例, 因为联表查询全部并行运行, 导致t1表的索引失效. 而t1表的是最庞大的表, 约有50万行数据, 而其他表只是几百行的配置信息表.

t1表的索引是性能的关键.

为了能利用上t1表的索引, 使用子查询, 先执行完t1,再去匹配信息. 

这种思路应该对各种业务导向的报表查询都有帮助, 因为要输出对人友好的结果.

下面的命令执行时间3.6s

SELECT
    t4.orgName,
    t3.projectName,
    t3.Partner,
    t1.type,
    t1.count
FROM
    (
        SELECT
            orgid,
            type,
            COUNT(DISTINCT imei) AS count
        FROM
            `t_teminal`
        GROUP BY
            orgid
    ) AS t1
LEFT JOIN `t_orgcode_orgid_rela` AS t2 ON t1.orgid = t2.orgId
LEFT JOIN `t_org_code` AS t3 ON t2.topOrganizationCode = t3.topOrganizationCode
LEFT JOIN `t_organization` AS t4 ON t1.orgid = t4.orgId

　　

http://www.cnblogs.com/oDoraemon/p/9001326.html