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，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 */
