mysql - Mysql子查询很慢

Question

我有这个 MySQL 问题。它工作正常，但速度极慢。运行最多需要 30 秒。

SELECT t.Id, Subject, k.info
FROM tip t
LEFT JOIN comments k ON t.Id = k.Tipid
WHERE event = 1
AND k.Id
IN (
SELECT Max(Id) FROM comments GROUP BY Tipid
)

如果我只运行子查询 ( SELECT Max(Id) FROM comments GROUP BY Tipid)，运行大约需要 0.02 秒。

子查询有问题。你有什么想法？

Answer 1

如果您选择 max(id)，则不需要 group by 试试这个

    SELECT t.Id, Subject, k.info
    FROM tip t
    LEFT JOIN comments k ON t.Id = k.Tipid
    WHERE event = 1
    AND k.Id in (select Max(Id) from comments)

Answer 2

试试这个：

SELECT t.Id, t.Subject, c.info
FROM tip t
LEFT JOIN
(
   SELECT Tipid, MAX(Id) AS MaxId
   FROM comments 
   GROUP BY Tipid
) AS c  ON t.Id = c.Tipid
       AND t.Id = c.MaxId
WHERE t.event = 1;

Answer 3

这有很多原因......我想你试试这个......

SELECT t.Id, Subject, k.info
FROM tip t
INNER JOIN comments k ON t.Id = k.Tipid
WHERE event = 1
  AND k.Id IN ( SELECT Max(Id) FROM comments GROUP BY Tipid )

因为 LEFT JOIN 在 50 条记录后变得太慢。