[英]MySQL Query is slow (Joins/counts)
我有这个查询:
SELECT `assemblies`.`id`,
`assemblies`.`type`,
`assemblies`.`champion`,
`assemblies`.`name`,
`assemblies`.`author`,
`assemblies`.`githublastmod`,
( assemblies.forum IS NOT NULL ) AS forumExists,
Count(votes.id) AS votesCount,
Count(install_clicks.id) AS installCount,
Count(github_clicks.id) AS githubCount,
Count(forum_clicks.id) AS forumCount
FROM `assemblies`
INNER JOIN `votes`
ON `votes`.`assembly` = `assemblies`.`id`
INNER JOIN `install_clicks`
ON `install_clicks`.`assembly` = `assemblies`.`id`
INNER JOIN `github_clicks`
ON `github_clicks`.`assembly` = `assemblies`.`id`
INNER JOIN `forum_clicks`
ON `forum_clicks`.`assembly` = `assemblies`.`id`
WHERE `assemblies`.`type` = 'utility'
AND Unix_timestamp(Date(assemblies.githublastmod)) > '1419536536'
GROUP BY `assemblies`.`id`
ORDER BY `votescount` DESC,
`githublastmod` DESC
由于某种原因,该查询非常慢,因此我正在使用数据库引擎MyISAM。 我希望有人可以在这里帮助我:)
说明命令:
您的问题应该使用正确的索引解决:
CREATE INDEX index_name_1 ON `votes`(`assembly`);
CREATE INDEX index_name_2 ON `install_clicks`(`assembly`);
CREATE INDEX index_name_3 ON `github_clicks`(`assembly`);
CREATE INDEX index_name_4 ON `forum_clicks`(`assembly`);
创建这些索引后,请再次尝试查询,它应该会更快。
我相信在这种情况下,对计数进行子查询将使其运行更快(并且值将是正确的)。
原始查询的问题是中间行的数量激增:对于每个“程序集”,有n1票,n2安装等。这导致每个程序集n1 * n2 * ...行。
SELECT `assemblies`.`id`, `assemblies`.`type`, `assemblies`.`champion`,
`assemblies`.`name`, `assemblies`.`author`, `assemblies`.`githublastmod`,
( assemblies.forum IS NOT NULL ) AS forumExists,
( SELECT Count(*)
FROM votes
WHERE `assembly` = `assemblies`.`id`
) AS votesCount,
( SELECT Count(*)
FROM install_clicks
WHERE `assembly` = `assemblies`.`id`
) AS installCount,
( SELECT Count(*)
FROM github_clicks
WHERE `assembly` = `assemblies`.`id`
) AS githubCount,
( SELECT Count(*)
FROM forum_clicks.id
WHERE `assembly` = `assemblies`.`id`
) AS forumCount
FROM `assemblies`
WHERE `assemblies`.`type` = 'utility'
AND Unix_timestamp(Date(assemblies.githublastmod)) > '1419536536'
ORDER BY `votescount` DESC, `githublastmod` DESC
每个辅助表都需要一个以assembly
开头的INDEX。
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