MySQL 中带有 GROUP BY 的子查询非常慢

Question

我知道了 SQL:

SELECT COUNT( d.id ) AS sum, d.user_agent
FROM debug AS d
WHERE d.id IN (
    SELECT MAX(d2.id)
    FROM debug AS d2
    GROUP BY d2.user_id
)
GROUP BY d.user_agent

我想获取相似用户代理的数量，但只需要每个用户最后一个条目的用户代理数量。

=> 计数(d.id)

=> 分组依据 d2.user_id

我尝试将两个 SQL 分开，并将子查询的响应写在另一个查询中。执行时间远低于一秒。两者一起约 25 秒。

我的table结构：

CREATE TABLE `debug` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `event` varchar(127) NOT NULL,
  `user_id` int(11) NOT NULL,
  `user_agent` varchar(255) NOT NULL,
  `timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `parameters` varchar(5000) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `user_id` (`user_id`),
  KEY `user_agent` (`user_agent`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;

SQL-Fiddle: Here

有什么优化方法吗？

Answer 1

这样更好吗？

SELECT x.user_agent
     , COUNT(*) total
  FROM debug x
  JOIN 
     ( SELECT user_id
            , MAX(id) max_id
         FROM debug 
        GROUP
           BY user_id
     ) y
    ON y.user_id = x.user_id
   AND y.max_id = x.id
 GROUP BY user_agent;

MySQL 中带有 GROUP BY 的子查询非常慢

Very slow subquery in MySQL with GROUP BY

mysql

group-by

subquery