一、概述
这是我们SQL优化班的一个学员，据说该SQL在生产环境中已经运行了20个小时，快把服务器的磁盘资源耗尽了。这20个小时，我们可爱的学员就是靠着删除一些不重要的文件才能够勉强度过。
据了解，该SQL为一个月运行一次的跑报表的SQL，主要问题是随着SQL的运行时间越来越长，所需的临时表空间也越来越大，导致磁盘资源用尽。
二、先看慢SQL
● SQL 执行时长 ，目前已经20小时，还在继续执行-
SELECT -
COUNT( * ) AS totalNum, -
sum( CASE WHEN F.ALARM_LEVEL = 1 THEN 1 ELSE 0 END ) AS LEVELS1, -
sum( CASE WHEN F.ALARM_LEVEL = 2 THEN 1 ELSE 0 END ) AS LEVELS2, -
sum( CASE WHEN F.ALARM_LEVEL = 3 THEN 1 ELSE 0 END ) AS LEVELS3, -
sum( CASE WHEN F.DEAL_STATE = 0 THEN 1 ELSE 0 END ) AS DESTS -
FROM -
F -
LEFT JOIN DC ON DC.ID = F.CONST_ID -
LEFT JOIN V ON V.ID = F.VEHICLE_ID -
LEFT JOIN AREA ON AREA.ID = V.SYS_DIVISION_ID -
WHERE -
DC.ID IS NOT NULL -
AND V.ID IS NOT NULL -
AND F.DEAL_STATE = 0 -
AND ALARM_LEVEL IN ( 1, 2, 3 ) -
AND F.VEHICLE_ID IN ( -
SELECT -
VEHICLE_ID -
FROM -
GVLK -
WHERE -
GROUP_ID IN ( SELECT GROUP_ID FROM GULK WHERE USER_ID = 'ff8080816091b09c0161f9b825750a9a' ) -
UNION -
SELECT -
VEHICLE_ID -
FROM -
UVLK -
WHERE -
USER_ID = 'ff8080816091b09c0161f9b825750a9a' -
) -
AND date( F.ALARM_TIME ) BETWEEN '2000-01-01' -
AND '2018-08-14' -
AND AREA.PATH LIKE CONCAT( ( SELECT ARE.PATH FROM ARE WHERE ARE.ID = '0' ), '%' )
上述SQL的含义：将F左连DC ，V，AREA表的结果进行where过滤，where中存在子查询，并且还有like函数
● 查看执行计划
-
SELECT -
COUNT( * ) AS totalNum -
FROM -
F -
WHERE F.DEAL_STATE = 0 -
AND ALARM_LEVEL IN ( 1, 2, 3 ) -
AND F.ALARM_TIME BETWEEN '2000-01-01' AND '2018-08-14'
上面的sql 运行10s 结果集为393653条数据，说明where 条件中的过滤条件的选择率不是很好 不适合创建索引。
● 验证子查询的过滤性-
SELECT -
VEHICLE_ID -
FROM -
GVLK -
WHERE GROUP_ID IN ( SELECT GROUP_ID FROM GULK WHERE USER_ID = 'ff8080816091b09c0161f9b825750a9a' ) -
UNION -
SELECT -
VEHICLE_ID -
FROM -
UVLK -
WHERE USER_ID = 'ff8080816091b09c0161f9b825750a9a'
上面的SQL 运行了0.5s 结果为1200条 。in里的结果速度快 结果集很小 ，F表 就该结果进行in操作，也会有大幅度的过滤。
● DEPEND SUBQUERY 改写-
SELECT -
COUNT( * ) AS totalNum -
FROM -
F -
join ( -
SELECT -
VEHICLE_ID -
FROM -
GVLK -
WHERE -
GROUP_ID IN ( SELECT GROUP_ID FROM GULK WHERE USER_ID = 'ff8080816091b09c0161f9b825750a9a' ) -
UNION -
SELECT -
VEHICLE_ID -
FROM -
UVLK -
WHERE -
USER_ID = 'ff8080816091b09c0161f9b825750a9a' -
) s -
WHERE F.DEAL_STATE = 0 -
AND ALARM_LEVEL IN ( 1, 2, 3 ) -
AND F.VEHICLE_ID = s.VEHICLE_ID -
AND F.ALARM_TIME BETWEEN '2000-01-01' AND '2018-08-14'
最后运行了如下sql 结果集为 88696 速度为0.5s。采用join的方式替代in的方式，因为 DEPEND SUBQUERY是依赖于SQL的主体部分，执行的次数与被依赖表结果集一致。
四、SQL优化结果
-
explain extended -
SELECT -
COUNT( * ) AS totalNum, -
sum( CASE WHEN F.ALARM_LEVEL = 1 THEN 1 ELSE 0 END ) AS LEVELS1, -
sum( CASE WHEN F.ALARM_LEVEL = 2 THEN 1 ELSE 0 END ) AS LEVELS2, -
sum( CASE WHEN F.ALARM_LEVEL = 3 THEN 1 ELSE 0 END ) AS LEVELS3, -
sum( CASE WHEN F.DEAL_STATE = 0 THEN 1 ELSE 0 END ) AS DESTS -
FROM -
F -
straight_join ( -
SELECT -
VEHICLE_ID -
FROM -
GVLK -
WHERE -
GROUP_ID IN ( SELECT GROUP_ID FROM GULK WHERE USER_ID = 'ff8080816091b09c0161f9b825750a9a' ) -
UNION -
SELECT -
VEHICLE_ID -
FROM -
UVLK -
WHERE -
USER_ID = 'ff8080816091b09c0161f9b825750a9a' -
) s on F.VEHICLE_ID = s.VEHICLE_ID -
straight_join DC ON DC.ID = F.CONST_ID -
straight_join V ON V.ID = F.VEHICLE_ID -
straight_join AREA ON AREA.ID = V.SYS_DIVISION_ID -
WHERE -
DC.ID IS NOT NULL -
AND V.ID IS NOT NULL -
AND F.DEAL_STATE = 0 -
AND ALARM_LEVEL IN ( 1, 2, 3 ) -
AND date( F.ALARM_TIME ) BETWEEN '2000-01-01' -
AND '2018-08-14' -
AND AREA.PATH LIKE CONCAT( ( SELECT ARE.PATH FROM ARE WHERE ARE.ID = '0' ), '%' )
1、使用join 代替in的方式；
2、修改后的语句可以使用到索引，索引为F.const_id，table为。这里值得一提的是，采用straight_join 代替 了 join，保证了SQL执行顺序一定是按照我们SQL书写的顺序。
四、后记
本文主要在于优化DEPEND SUBQUERY,另外让SQL能够用得上索引，让SQL的速度有着显著的提升。
原文发布时间为：2018-10-31
本文作者：郑松华老师&小鹿
