[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join查询的实现原理[MySQL 的Join及底层实现原理], 导致驱动表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.

所以利用t1表的索引是性能的关键.

由于统计中有groupby and count的需求，通过t1表即可实现, 顺势使用了子查询, 先处理完t1,再去匹配信息. 

这种思路很简单，但应该对各种业务导向的报表查询都有帮助,.

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