Question

我有这个MySQL问题。它工作正常，但它非常慢。最多需要30秒才能运行。

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），则运行大约需要0.02秒。

子查询有问题。你有什么想法吗？

Answer 1

如果你选择max（id）试试这个

，你不需要分组

    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

请改为尝试：

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条记录后变得太慢了。

Mysql子查询很慢

3 个答案: