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 */

参考资料

• MySQL performance optimization: order by datetime field