MySQL优化 exists/in改写join

问题语句

SELECT * FROM a 
WHERE `type` = 'appointment' 
AND `event` = 14 
AND EXISTS ( 
SELECT * FROM b 
WHERE a.`sheet_id` = b.`id` 
AND `company_id` = 8 
AND b.`deleted_at` IS NULL ) 
ORDER BY a.id DESC 
LIMIT 6;

解读执行计划

在exists类型的子查询的执行计划中，select_type一栏分别是PRIMARY和DEPENDENT SUBQUERY

DEPENDENT SUBQUERY的意思是：子查询，依赖于外层的查询;DEPEND SUBQUERY是依赖于SQL的主体部分，它的执行次数最大可能和SQL主体部分结果的行数一样多（这里因为有limit6，所以看起来主表扫描行数是6，如果去掉这个limit6的话，这个值是1500W）

上面这句话解释得通俗一点就是外连接先执行查询，然后把查询的结果集放入子查询内进行匹配；外查询每执行一次查询，就要来子查询匹配一次

join的执行计划中，select_type一栏都是simple

join的第一行的就是外表

从上面的对比可以看出，无论是 exists类型的子查询 还是join，都基本可以看作遵循了第一行就是驱动表的规则（注意不是所有子查询都遵循这个规则，本篇只针对exists类型的dependent subquery）

分析

图一PRIMARY对应的表就是图二中的a表，DEPENDENT SUBQUERY表就是图二中的b表；a表有1500W行数据，b表有2W行数据

所以图一的SQL执行效率如此低下的原因就是大表驱动小表

优化

exists改写为join

1 SELECT a.*
2 FROM  a join b on a.`sheet_id` = b.`id`
3 WHERE a.`type` = 'appointment'
4 AND a.`event` = 14
5 AND b.`company_id` = 8
6 AND b.`deleted_at` IS NULL
7 ORDER BY a.`id` DESC
8 LIMIT 6;

由于a表作为内表，因此在a.`sheet_id`，a.`type`，a.`event`上创建联合索引；语句中出现了b表的本地谓词，所以b表的b.`company_id`，b.`deleted_at`上也要创建联合索引

优化结果，执行时间：117s→0.36s，性能提升了2000倍

这个语句有一个更极端的取值，在b.`company_id` = 2的时候，小表不会搜出任何满足条件的结果，在这种情况下，原语句执行时间在350s以上，而新语句仅需要0.03s，性能提升万倍

优化案例

今天优化的这批语句中，大多数是exists子查询的问题，可以看出这个研发小哥非常的喜欢用exists这种写法；前面的那个exists语句是泛用型，后面的exists语句加了些新花样

eg.

 1 SELECT SUM(`xxxx`) AS ag
 2 FROM a
 3 WHERE EXISTS (
 4 SELECT * FROM b
 5 WHERE a.`delivery_sheet_id` = b.`id`
 6 AND (`status` = 4
 7 OR `is_rejected` = '1')
 8 AND `company_id` = 8
 9 AND b.`deleted_at` IS NULL
10 )
11 AND `status` IN (0, 4)
12 AND `collection_type` IN (2, 3)
13 AND a.`deleted_at` IS NULL;

or的优化通常改写union，但这里是求sum不能这么改，需要改写成2个语句然后求和；对应的列要建好索引

 1 select c.ag+d.ag as ag from
 2 (SELECT SUM(a.`xxxx`) AS ag
 3 FROM  a join b
 4 on a.`delivery_sheet_id` = b.`id`
 5 where
 6 b.`status` = 4
 7 AND    b.`company_id` = 8
 8 AND b.`deleted_at` IS NULL
 9 AND a.`status` IN (0, 4)
10 AND a.`collection_type` IN (2, 3)
11 AND a.`deleted_at` IS NULL) c,
12 (
13 SELECT SUM(a.`xxxx`) AS ag
14 FROM a join b
15 on a.`delivery_sheet_id` = b.`id`
16 where
17 b.`is_rejected` = '1'
18 AND    b.`company_id` = 8
19 AND b.`deleted_at` IS NULL
20 AND a.`status` IN (0, 4)
21 AND a.`collection_type` IN (2, 3)
22 AND a.`deleted_at` IS NULL) d;

优化结果，执行时间：18s→0.2s

in改写join的思路和exists差不多

这里没有现成的例子，粘贴一篇郑松华老师公众号的分析过来

原语句

 1 SELECT
 2 
 3 COUNT( * ) AS totalNum,
 4 
 5 sum( CASE WHEN F.ALARM_LEVEL = 1 THEN 1 ELSE 0 END ) AS LEVELS1,
 6 
 7 sum( CASE WHEN F.ALARM_LEVEL = 2 THEN 1 ELSE 0 END ) AS LEVELS2,
 8 
 9 sum( CASE WHEN F.ALARM_LEVEL = 3 THEN 1 ELSE 0 END ) AS LEVELS3,
10 
11 sum( CASE WHEN F.DEAL_STATE = 0 THEN 1 ELSE 0 END ) AS DESTS
12 
13 FROM
14 
15 F
16 
17 LEFT JOIN  DC ON DC.ID = F.CONST_ID
18 
19 LEFT JOIN  V ON V.ID = F.VEHICLE_ID
20 
21 LEFT JOIN  AREA ON AREA.ID = V.SYS_DIVISION_ID
22 
23 WHERE
24 
25 DC.ID IS NOT NULL
26 
27 AND V.ID IS NOT NULL
28 
29 AND F.DEAL_STATE = 0
30 
31 AND ALARM_LEVEL IN ( 1, 2, 3 )
32 
33 AND F.VEHICLE_ID IN (
34 
35 SELECT
36 
37 VEHICLE_ID
38 
39 FROM
40 
41 GVLK
42 
43 WHERE
44 
45 GROUP_ID IN ( SELECT GROUP_ID FROM GULK WHERE USER_ID = 'ff8080816091b09c0161f9b825750a9a' )
46 
47 UNION
48 
49 SELECT
50 
51 VEHICLE_ID
52 
53 FROM
54 
55 UVLK
56 
57 WHERE
58 
59 USER_ID = 'ff8080816091b09c0161f9b825750a9a'
60 
61 )
62 
63 AND date( F.ALARM_TIME ) BETWEEN '2000-01-01'
64 
65 AND '2018-08-14'
66 
67 AND AREA.PATH LIKE CONCAT( ( SELECT ARE.PATH FROM  ARE WHERE ARE.ID = '0' ), '%' )

执行计划如下

改写如下（in改join）

 1 explain extended
 2 
 3 SELECT
 4 
 5 COUNT( * ) AS totalNum,
 6 
 7 sum( CASE WHEN F.ALARM_LEVEL = 1 THEN 1 ELSE 0 END ) AS LEVELS1,
 8 
 9 sum( CASE WHEN F.ALARM_LEVEL = 2 THEN 1 ELSE 0 END ) AS LEVELS2,
10 
11 sum( CASE WHEN F.ALARM_LEVEL = 3 THEN 1 ELSE 0 END ) AS LEVELS3,
12 
13 sum( CASE WHEN F.DEAL_STATE = 0 THEN 1 ELSE 0 END ) AS DESTS
14 
15 FROM
16 
17 F
18 
19 straight_join (
20 
21 SELECT
22 
23 VEHICLE_ID
24 
25 FROM
26 
27 GVLK
28 
29 WHERE
30 
31 GROUP_ID IN ( SELECT GROUP_ID FROM GULK WHERE USER_ID = 'ff8080816091b09c0161f9b825750a9a' )
32 
33 UNION
34 
35 SELECT
36 
37 VEHICLE_ID
38 
39 FROM
40 
41 UVLK
42 
43 WHERE
44 
45 USER_ID = 'ff8080816091b09c0161f9b825750a9a'
46 
47 ) s on F.VEHICLE_ID = s.VEHICLE_ID
48 
49 straight_join DC ON DC.ID = F.CONST_ID
50 
51 straight_join V ON V.ID = F.VEHICLE_ID
52 
53 straight_join AREA ON AREA.ID = V.SYS_DIVISION_ID
54 
55 WHERE
56 
57 DC.ID IS NOT NULL
58 
59 AND V.ID IS NOT NULL
60 
61 AND F.DEAL_STATE = 0
62 
63 AND ALARM_LEVEL IN ( 1, 2, 3 )  
64 
65 AND date( F.ALARM_TIME ) BETWEEN '2000-01-01'
66 
67 AND '2018-08-14'
68 
69 AND AREA.PATH LIKE CONCAT( ( SELECT ARE.PATH FROM ARE WHERE ARE.ID = '0' ), '%' )

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