MySQL 8.0窗口函数优化SQL一例

1. 问题描述

最近在折腾把所有mysql slow query log写入到数据库中,再集中展示,向业务部门开放,也方便业务部门的同学自行查看并优化各自业务内的慢SQL。增加了定期生成报表的功能,统计最近1~2周内的慢查询数量变化情况,给业务方同学更直观的数据对比,了解最近这段时间的慢查询数量变化情况,是多了还是少了。于是有了下面这一坨SQL:

select hostname_max , db_max, sum(ts_cnt) as 1W
(select ifnull(sum(t1.ts_cnt),0) as ts_cnt from global_query_review_history t1 where 
t1.hostname_max=t2.hostname_max and t1.ts_min>= date_sub(now(), interval 14 day) and 
t1.ts_max<= date_sub(now(), interval 7 day)) AS 2W 
from global_query_review_history t2 where 
ts_min>= date_sub(now(), interval 7 day) 
group by hostname_max, db_max 
order by 1W desc limit 20;

当前 global_query_review_history 表约有2.5万条记录,这条SQL耗时 1.16秒,显然太慢了。下面是SQL执行计划:

*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: t2
   partitions: NULL
         type: ALL
possible_keys: ts_min
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 25198
     filtered: 41.09
        Extra: Using where; Using temporary; Using filesort
*************************** 2. row ***************************
           id: 2
  select_type: DEPENDENT SUBQUERY
        table: t1
   partitions: NULL
         type: ref
possible_keys: hostname_max,ts_min
          key: hostname_max
      key_len: 258
          ref: func
         rows: 20
     filtered: 14.90
        Extra: Using where

可以看到需要进行一次子查询(无法自动优化成JOIN)。

SQL执行后的status统计值:

+-----------------------+--------+
| Variable_name         | Value  |
+-----------------------+--------+
| Handler_read_first    | 0      |
| Handler_read_key      | 17328  |
| Handler_read_last     | 0      |
| Handler_read_next     | 809121 |
| Handler_read_prev     | 0      |
| Handler_read_rnd      | 0      |
| Handler_read_rnd_next | 25380  |
+-----------------------+--------+

可以看到除了有全表扫描外,还要根据索引的多次逐行扫描(Handler_read_next = 809121,子查询引起的)。

2. SQL优化

上面的SQL主要瓶颈在于嵌套子查询,去掉子查询,即便是全表扫描也还是很快的。

[root@yejr.run]> select ...
...
20 rows in set (0.08 sec)

[root@yejr.run]> show status like 'handler%read%';
+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| Handler_read_first    | 0     |
| Handler_read_key      | 16910 |
| Handler_read_last     | 0     |
| Handler_read_next     | 0     |
| Handler_read_prev     | 0     |
| Handler_read_rnd      | 0     |
| Handler_read_rnd_next | 25380 |
+-----------------------+-------+

SQL优化有困难自然先想到了松华老师,在得知我用的MySQL 8.0之后,他帮忙给改造成了基于窗口函数的写法:

select hostname_max , db_max,
sum( case when ts_min>= date_sub(now(), interval 7 day)  then ts_cnt end ) as 1W,
ifnull(sum(case when  ts_min>= date_sub(now(), interval 14 day)
   and ts_max<= date_sub(now(), interval 7 day) then ts_cnt end ) over(partition by hostname_max),0) 2W
from global_query_review_history t2
 where ts_min>= date_sub(now(), interval 14 day)
group by hostname_max, db_max
order by 1W desc limit 20;

再看下执行计划:

*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t2
   partitions: NULL
         type: ALL
possible_keys: ts_min
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 25198
     filtered: 44.88
        Extra: Using where; Using temporary; Using filesort

新SQL比较取巧,只需要读取一次数据,利用窗口函数直接计算出需要的统计值。虽然有可用索引,但因为要扫描的数据量比较大,所以最后还是变成全表扫描。新SQL耗时和status统计值见下:

20 rows in set (0.08 sec)

[root@yejr.run]> show status like 'handler%read%';
+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| Handler_read_first    | 0     |
| Handler_read_key      | 24396 |
| Handler_read_last     | 0     |
| Handler_read_next     | 0     |
| Handler_read_prev     | 0     |
| Handler_read_rnd      | 886   |
| Handler_read_rnd_next | 26703 |
+-----------------------+-------+

和之前那个SQL差距太大了,优化效果杠杠滴。

全文完。

Enjoy MySQL 8.0 :)

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