mysql时间排序_MySQL性能优化：按日期时间字段排序
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我有一张大约有100000个博客文章的表格，通过1：n关系链接到一个有50个Feed的表格。当我用一个select语句查询这两个表时，由发布表的日期时间字段排序，MySQL总是使用filesort，导致查询时间非常慢(> 1秒)。以下是postings表(简化)的模式： +---------------------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +---------------------+--------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | feed_id | int(11) | NO | MUL | NULL | | | crawl_date | datetime | NO | | NULL | | | is_active | tinyint(1) | NO | MUL | 0 | | | link | varchar(255) | NO | MUL | NULL | | | author | varchar(255) | NO | | NULL | | | title | varchar(255) | NO | | NULL | | | excerpt | text | NO | | NULL | | | long_excerpt | text | NO | | NULL | | | user_offtopic_count | int(11) | NO | MUL | 0 | | +---------------------+--------------+------+-----+---------+----------------+ 这是feed表： +-------------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------------+--------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | type | int(11) | NO | MUL | 0 | | | title | varchar(255) | NO | | NULL | | | website | varchar(255) | NO | | NULL | | | url | varchar(255) | NO | | NULL | | +-------------+--------------+------+-----+---------+----------------+ 以下是执行时间大于1秒的查询。请注意，post_date字段有一个索引，但是MySQL不使用它来对发布表进行排序： SELECT `postings`.`id`, UNIX_TIMESTAMP(postings.post_date) as post_date, `postings`.`link`, `postings`.`title`, `postings`.`author`, `postings`.`excerpt`, `postings`.`long_excerpt`, `feeds`.`title` AS feed_title, `feeds`.`website` AS feed_website FROM (`postings`) JOIN `feeds` ON `feeds`.`id` = `postings`.`feed_id` WHERE `feeds`.`type` = 1 AND `postings`.`user_offtopic_count` < 10 AND `postings`.`is_active` = 1 ORDER BY `postings`.`post_date` desc LIMIT 15 这个查询的explain extended命令的结果显示MySQL正在使用filesort： +----+-------------+----------+--------+---------------------------------------+-----------+---------+--------------------------+-------+-----------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+--------+---------------------------------------+-----------+---------+--------------------------+-------+-----------------------------+ | 1 | SIMPLE | postings | ref | feed_id,is_active,user_offtopic_count | is_active | 1 | const | 30996 | Using where; Using filesort | | 1 | SIMPLE | feeds | eq_ref | PRIMARY,type | PRIMARY | 4 | feedian.postings.feed_id | 1 | Using where | +----+-------------+----------+--------+---------------------------------------+-----------+---------+--------------------------+-------+-----------------------------+ 当我删除order by部分时，MySQL停止使用filesort。求帮忙，如果你有任何想法如何优化这个查询让MySQL排序和选择使用索引的数据。我已经尝试了一些东西mysql排序，比如像在一些博客帖子中提到的那样，在所有的字段/字段上创建一个组合索引，但是这也不起作用。 最佳解决思路 在postings (is_active, post_date)上创建一个复合索引(按此顺序)。 它将用于is_active的过滤和post_date的排序。 MySQL应在EXPLAIN EXTENDED中显示REF访问方法。 请注意，通过user_offtopic_count您有一个RANGE过滤条件，这就是为什么您不能使用该字段的索引在过滤和其他字段进行排序。 根据user_offtopic_count的选择性(即多少行满足user_offtopic_count < 10)的不同，在user_offtopic_count上创建索引并对post_dates进行排序可能会更有用。 为此，请在postings (is_active, user_offtopic_count)上创建一个复合索引，并确保使用了通过此索引的RANGE访问方法。 哪个索引将更快取决于您的数据分布。创建两个索引，FORCE他们，看看哪个更快： CREATE INDEX ix_active_offtopic ON postings (is_active, user_offtopic_count); CREATE INDEX ix_active_date ON postings (is_active, post_date); SELECT `postings`.`id`, UNIX_TIMESTAMP(postings.post_date) as post_date, `postings`.`link`, `postings`.`title`, `postings`.`author`, `postings`.`excerpt`, `postings`.`long_excerpt`, `feeds`.`title` AS feed_title, `feeds`.`website` AS feed_website FROM `postings` FORCE INDEX (ix_active_offtopic) JOIN `feeds` ON `feeds`.`id` = `postings`.`feed_id` WHERE `feeds`.`type` = 1 AND `postings`.`user_offtopic_count` < 10 AND `postings`.`is_active` = 1 ORDER BY `postings`.`post_date` desc LIMIT 15 /* This should show RANGE access with few rows and keep the FILESORT */ SELECT `postings`.`id`, UNIX_TIMESTAMP(postings.post_date) as post_date, `postings`.`link`, `postings`.`title`, `postings`.`author`, `postings`.`excerpt`, `postings`.`long_excerpt`, `feeds`.`title` AS feed_title, `feeds`.`website` AS feed_website FROM `postings` FORCE INDEX (ix_active_date) JOIN `feeds` ON `feeds`.`id` = `postings`.`feed_id` WHERE `feeds`.`type` = 1 AND `postings`.`user_offtopic_count` < 10 AND `postings`.`is_active` = 1 ORDER BY `postings`.`post_date` desc LIMIT 15 /* This should show REF access with lots of rows and no FILESORT */
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