mysql - 使用Explain调试缓慢的mySQL查询

Question

在我们的系统中发现了低效的查询。 content 保存幻灯片的版本，这应该通过 id 选择幻灯片的最高版本。

SELECT `content`.*
FROM (`content`)
JOIN (
SELECT max(version) as `version` from `content`
WHERE `slide_id` = '16901'
group by `slide_id`
) c ON `c`.`version` = `content`.`version`;

解释一下

+----+-------------+------------------+------------+--------+--------------------------------------------------------------------------------+------------------------------------+---------+-------+------+----------+--------------------------+
| id | select_type | table            | partitions | type   | possible_keys                                                                  | key                                | key_len | ref   | rows | filtered | Extra                    |
+----+-------------+------------------+------------+--------+--------------------------------------------------------------------------------+------------------------------------+---------+-------+------+----------+--------------------------+
|  1 | PRIMARY     | <derived2>       | NULL       | system | NULL                                                                           | NULL                               | NULL    | NULL  |    1 |   100.00 | NULL                     |
|  1 | PRIMARY     | content          | NULL       | ref    | PRIMARY,version                                                                | PRIMARY                            | 8       | const | 9703 |   100.00 | NULL                     |
|  2 | DERIVED     | content          | NULL       | ref    | PRIMARY,fk_content_slides_idx,thumbnail_asset_id,version,slide_id                      | fk_content_slides_idx              | 8       | const |    1 |   100.00 | Using where; Using index |
+----+-------------+------------------+------------+--------+--------------------------------------------------------------------------------+------------------------------------+---------+-------+------+----------+--------------------------+

一个大问题是它返回系统中几乎所有幻灯片，因为外部查询不按幻灯片 ID 进行过滤。添加后我得到...

SELECT `content`.* 
FROM (`content`) 
JOIN ( 
SELECT max(version) as `version` from `content`  
WHERE `slide_id` = '16901' group by `slide_id` 
) c ON `c`.`version` = `content`.`version` 
WHERE `slide_id` = '16901';

解释一下

+----+-------------+------------------+------------+--------+--------------------------------------------------------------------------------+------------------------------------+---------+-------------+------+----------+--------------------------+
| id | select_type | table            | partitions | type   | possible_keys                                                                  | key                                | key_len | ref         | rows | filtered | Extra                    |
+----+-------------+------------------+------------+--------+--------------------------------------------------------------------------------+------------------------------------+---------+-------------+------+----------+--------------------------+
|  1 | PRIMARY     | <derived2>       | NULL       | system | NULL                                                                           | NULL                               | NULL    | NULL        |    1 |   100.00 | NULL                     |
|  1 | PRIMARY     | content          | NULL       | const  | PRIMARY,fk_content_slides_idx,version,slide_id                                 | PRIMARY                            | 16      | const,const |    1 |   100.00 | NULL                     |
|  2 | DERIVED     | content          | NULL       | ref    | PRIMARY,fk_content_slides_idx,thumbnail_asset_id,version,slide_id              | fk_content_slides_idx              | 8       | const       |    1 |   100.00 | Using where; Using index |
+----+-------------+------------------+------------+--------+--------------------------------------------------------------------------------+------------------------------------+---------+-------------+------+----------+--------------------------+

这可以正确地将行数减少到一，但并没有真正加快速度。

版本、slide_id 都有索引，版本和 Slide_id 上有唯一键。

我还能做些什么来加快速度吗？

使用 TOP LIMIT 1 代替 Max ？

米

Answer 1

MySQL 似乎采用索引(版本、slide_id)来连接表。您应该获得更好的结果

SELECT `content`.* 
FROM `content`
FORCE INDEX FOR JOIN (fk_content_slides_idx) 
join (    
    SELECT `slide_id`, max(version) as `version` from `content`  
    WHERE `slide_id` = '16901' group by `slide_id` 
) c ON `c`.`slide_id` = `content`.`slide_id` and `c`.`version` = `content`.`version` 

您需要一个以 Slide_id 作为第一列的索引，我只是猜测它是 fk_content_slides_idx，如果没有，请使用另一个索引。

FORCE INDEX FOR JOIN (fk_content_slides_idx) 部分只是为了强制执行它，您应该尝试mysql是否自行获取它而不强制(它应该)。

如果您发现差异，您可能会通过索引(slide_id、版本)获得稍微更好的结果，这取决于数据量(例如每个 id 的版本数)(但您不应该垃圾邮件索引，并且这张 table 上已经有很多东西了，但你可以尝试一下。)