[MySQL] MySQL聯表查詢的執行順序優化查詢

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

聯表查詢會涉及到多個表的操作, 在索引的利用上會受限制. 上面的命令執行時間720s

以上面為例, 因為join查詢的實現原理[MySQL 的Join及底層實現原理], 導致驅動表t1表的索引失效。而t1表的是最龐大的表, 約有50萬行數據, 而其他表只是幾百行的配置信息表.

所以利用t1表的索引是性能的關鍵.

由於統計中有groupby and count的需求，通過t1表即可實現, 順勢使用了子查詢, 先處理完t1,再去匹配信息. 

這種思路很簡單，但應該對各種業務導向的報表查詢都有幫助,.

下面的命令執行時間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