使用ORDER BY时，MySQL查询速度慢

Question

我有这个问题：

SELECT article.id
FROM article
INNER JOIN article_cCountry ON article.id = ID1 AND ID2 = 1
INNER JOIN user_cCountry ON article.uId = user_cCountry.ID1 AND user_cCountry.ID2 = 1
LEFT JOIN user ON article.uId = user.ID
WHERE article.released = "TRUE"
AND article.sDate < now()
AND article.cDate != "0000-00-00 00:00:00"
AND (article.eDate > now() OR article.eDate = 0)
AND ( (user.released = true) OR (article.uId = 0) )
ORDER BY article.cDate DESC
LIMIT 0, 10

查询需要约0.3秒，而ORDER BY只需约0.001秒。

为什么ORDER BY这么慢的想法？

修改 说明：

编辑2 指标

Answer 1

如果没有ORDER BY，您的查询将在10行（LIMIT）之后终止。

需要生成ORDER BY完整结果集，排序，然后返回前10行。

Answer 2

虽然你永远无法达到只输出10行的速度，然后对所有行进行排序，然后排序em然后输出前10行，你可以做一些小技巧来记录内存中的记录集

SELECT id FROM (
 SELECT article.id,article.cDate
 FROM article
 INNER JOIN article_cCountry ON article.id = ID1 AND ID2 = 1
 INNER JOIN user_cCountry ON article.uId = user_cCountry.ID1 AND user_cCountry.ID2 = 1
 LEFT JOIN user ON article.uId = user.ID
 WHERE article.released = "TRUE"
 AND article.sDate < now()
 AND article.cDate != "0000-00-00 00:00:00"
 AND (article.eDate > now() OR article.eDate = 0)
 AND ( (user.released = true) OR (article.uId = 0) )
)
ORDER BY cDate DESC
LIMIT 0, 10

希望有所帮助

Answer 3

INDEX(released, cDate)

may help

Avoiding OR may help.

Answer 4

Your ORDER BY column (article.cDate) should be indexed, this will stop the ORDER BY article.cDate taking a long time, as the column values will be indexed and then can be arranged exponentially faster.