MySQL 使用 JOIN 和 COUNT 进行查询优化

Question

我有以下 MySQL 查询：

SELECT t1.id, t1.releaseid, t1.site, t1.date, t2.pos FROM `tracers` as t1
LEFT JOIN (
    SELECT `releaseid`, `date`, COUNT(*) AS `pos` 
    FROM `tracers` GROUP BY `releaseid`
) AS t2 ON t1.releaseid = t2.releaseid AND t2.date <= t1.date 
ORDER BY `date` DESC , `pos` DESC LIMIT 0 , 100

想法是将 select 发布并计算在记录日期之前有多少其他网站也发布了它，以获得 position。

解释说：

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   PRIMARY t1  ALL NULL    NULL    NULL    NULL    498422  Using temporary; Using filesort
1   PRIMARY <derived2>  ALL NULL    NULL    NULL    NULL    91661    
2   DERIVED tracers index   NULL    releaseid   4   NULL    498422   

关于如何消除 Using 临时的任何建议； 使用文件排序？ 这需要很长时间。 我想到并尝试过的索引没有任何帮助。

Answer 1

下面的这个答案可能不会改变解释 output，但是如果您的主要问题是排序数据，它通过删除 order 子句确定将使您的查询运行得更快，请先尝试对您的子查询连接表进行排序，您的查询将是：

SELECT t1.id, t1.releaseid, t1.site, t1.date, t2.pos FROM `tracers` as t1
LEFT JOIN (
    SELECT `releaseid`, `date`, COUNT(*) AS `pos` 
    FROM `tracers` GROUP BY `releaseid`
    ORDER BY `pos` DESC -- additional order
) AS t2 ON t1.releaseid = t2.releaseid AND t2.date <= t1.date 
ORDER BY `date` DESC , `pos` DESC LIMIT 0 , 100

注意：我的数据库版本是 mysql-5.0.96-x64，也许在另一个版本中你会得到不同的结果。

Answer 2

尝试在tracers.date上添加一个索引，在tracers.releaseid上添加一个

Answer 3

1. 确保你有一个关于 releaseid 的索引。
2. 翻转您的 JOIN，子查询必须在 LEFT JOIN 的左侧。
3. 将 ORDER BY 和 LIMIT 子句放在子查询中。

Answer 4

尝试有两个索引，一个在(date)上，一个在(releaseid, date)上。

另一件事是您的查询似乎没有按照您的描述进行。 它实际上计算正确吗？

尝试将其重写为：

SELECT t1.id, t1.releaseid, t1.site, t1.`date`
     , COUNT(*) AS pos
FROM tracers AS t1
  JOIN tracers AS t2
    ON  t2.releaseid = t1.releaseid
    AND t2.`date` <= t1.`date` 
GROUP BY t1.releaseid
ORDER BY t1.`date` DESC
       , pos DESC
LIMIT 0 , 100

或作为：

SELECT t1.id, t1.releaseid, t1.site, t1.`date`
     , ( SELECT COUNT(*)
         FROM tracers AS t2
         WHERE t2.releaseid = t1.releaseid
           AND t2.`date` <= t1.`date`
       ) AS pos
FROM tracers AS t1
ORDER BY t1.`date` DESC
       , pos DESC
LIMIT 0 , 100