mysql - 尽管存在索引，但复杂的 MySQL 查询仍在使用文件排序

Question

我有一个包含数千行内容(大约 300 万行)的 Joomla 表。在查询表时，我在重写数据库查询以尽可能快时遇到了一些麻烦。

这是我的完整查询:

SELECT cc.title AS category, a.id, a.title, a.alias, a.title_alias, a.introtext, a.fulltext, a.sectionid, a.state, a.catid, a.created, a.created_by, a.created_by_alias, a.modified, a.modified_by, a.checked_out, a.checked_out_time, a.publish_up, a.publish_down, a.attribs, a.hits, a.images, a.urls, a.ordering, a.metakey, a.metadesc, a.access, CASE WHEN CHAR_LENGTH(a.alias) THEN CONCAT_WS(":", a.id, a.alias) ELSE a.id END AS slug, CASE WHEN CHAR_LENGTH(cc.alias) THEN CONCAT_WS(":", cc.id, cc.alias) ELSE cc.id END AS catslug, CHAR_LENGTH( a.`fulltext` ) AS readmore, u.name AS author, u.usertype, g.name AS groups, u.email AS author_email
FROM j15_content AS a
LEFT JOIN j15_categories AS cc
ON a.catid = cc.id
LEFT JOIN j15_users AS u
ON u.id = a.created_by
LEFT JOIN j15_groups AS g
ON a.access = g.id
WHERE 1
AND a.access <= 0
AND a.catid = 108
AND a.state = 1
AND ( publish_up = '0000-00-00 00:00:00' OR publish_up <= '2012-02-08 00:16:26' )
AND ( publish_down = '0000-00-00 00:00:00' OR publish_down >= '2012-02-08 00:16:26' )
ORDER BY a.title, a.created DESC
LIMIT 0, 10

这是来自 EXPLAIN 的输出:

 +----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-----------------------------+
| id | select_type | table | type   | possible_keys                                         | key       | key_len | ref                       | rows    | Extra                       |
+----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-----------------------------+
|  1 | SIMPLE      | a     | ref    | idx_access,idx_state,idx_catid,idx_access_state_catid | idx_catid | 4       | const                     | 3108187 | Using where; Using filesort |
|  1 | SIMPLE      | cc    | const  | PRIMARY                                               | PRIMARY   | 4       | const                     |       1 |                             |
|  1 | SIMPLE      | u     | eq_ref | PRIMARY                                               | PRIMARY   | 4       | database.a.created_by     |       1 |                             |
|  1 | SIMPLE      | g     | eq_ref | PRIMARY                                               | PRIMARY   | 1       | database.a.access         |       1 |                             |
+----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-----------------------------+

为了显示存在哪些索引，SHOW INDEX FROM j15_content:

+-------------+------------+------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table       | Non_unique | Key_name               | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------------+------------+------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| j15_content |          0 | PRIMARY                |            1 | id          | A         |     3228356 |     NULL | NULL   |      | BTREE      |         |
| j15_content |          1 | idx_section            |            1 | sectionid   | A         |           2 |     NULL | NULL   |      | BTREE      |         |
| j15_content |          1 | idx_access             |            1 | access      | A         |           1 |     NULL | NULL   |      | BTREE      |         |
| j15_content |          1 | idx_checkout           |            1 | checked_out | A         |           2 |     NULL | NULL   |      | BTREE      |         |
| j15_content |          1 | idx_state              |            1 | state       | A         |           2 |     NULL | NULL   |      | BTREE      |         |
| j15_content |          1 | idx_catid              |            1 | catid       | A         |           6 |     NULL | NULL   |      | BTREE      |         |
| j15_content |          1 | idx_createdby          |            1 | created_by  | A         |           1 |     NULL | NULL   |      | BTREE      |         |
| j15_content |          1 | title                  |            1 | title       | A         |      201772 |        4 | NULL   |      | BTREE      |         |
| j15_content |          1 | idx_access_state_catid |            1 | access      | A         |           1 |     NULL | NULL   |      | BTREE      |         |
| j15_content |          1 | idx_access_state_catid |            2 | state       | A         |           2 |     NULL | NULL   |      | BTREE      |         |
| j15_content |          1 | idx_access_state_catid |            3 | catid       | A         |           7 |     NULL | NULL   |      | BTREE      |         |
| j15_content |          1 | idx_title_created      |            1 | title       | A         |     3228356 |        8 | NULL   |      | BTREE      |         |
| j15_content |          1 | idx_title_created      |            2 | created     | A         |     3228356 |     NULL | NULL   |      | BTREE      |         |
+-------------+------------+------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

如您所见，从数据库中获取了一些数据。现在我通过简化查询来测试真正的问题在于 ORDER BY 子句。没有对结果进行排序，查询是相当敏感的，这里是一个解释:

+----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-------------+
| id | select_type | table | type   | possible_keys                                         | key       | key_len | ref                       | rows    | Extra       |
+----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-------------+
|  1 | SIMPLE      | a     | ref    | idx_access,idx_state,idx_catid,idx_access_state_catid | idx_catid | 4       | const                     | 3108187 | Using where |
|  1 | SIMPLE      | cc    | const  | PRIMARY                                               | PRIMARY   | 4       | const                     |       1 |             |
|  1 | SIMPLE      | u     | eq_ref | PRIMARY                                               | PRIMARY   | 4       | database.a.created_by     |       1 |             |
|  1 | SIMPLE      | g     | eq_ref | PRIMARY                                               | PRIMARY   | 1       | database.a.access         |       1 |             |
+----+-------------+-------+--------+-------------------------------------------------------+-----------+---------+---------------------------+---------+-------------+

如您所见，是致命的文件排序导致服务器崩溃。有了这么多行，我正在尽最大努力通过索引优化所有内容，但仍然有些问题。任何意见将不胜感激。

尝试使用 FORCE INDEX 无济于事:

explain     SELECT cc.title AS category, a.id, a.title, a.alias, a.title_alias, a.introtext, a.fulltext, a.sectionid, a.state, a.catid, a.created, a.created_by, a.created_by_alias, a.modified, a.modified_by, a.checked_out, a.checked_out_time, a.publish_up, a.publish_down, a.attribs, a.hits, a.images, a.urls, a.ordering, a.metakey, a.metadesc, a.access, CASE WHEN CHAR_LENGTH(a.alias) THEN CONCAT_WS(":", a.id, a.alias) ELSE a.id END AS slug, CASE WHEN CHAR_LENGTH(cc.alias) THEN CONCAT_WS(":", cc.id, cc.alias) ELSE cc.id END AS catslug, CHAR_LENGTH( a.`fulltext` ) AS readmore, u.name AS author, u.usertype, g.name AS groups, u.email AS author_email
    ->     FROM bak_content AS a
    ->     FORCE INDEX (idx_title_created)
    ->     LEFT JOIN bak_categories AS cc
    ->     ON a.catid = cc.id
    ->     LEFT JOIN bak_users AS u
    ->     ON u.id = a.created_by
    ->     LEFT JOIN bak_groups AS g
    ->     ON a.access = g.id
    ->     WHERE 1
    ->     AND a.access <= 0
    ->     AND a.catid = 108
    ->     AND a.state = 1
    ->     AND ( publish_up = '0000-00-00 00:00:00' OR publish_up <= '2012-02-08
    ->     AND ( publish_down = '0000-00-00 00:00:00' OR publish_down >= '2012-0
    ->     ORDER BY a.title, a.created DESC
    ->     LIMIT 0, 10;

生产:

+----+-------------+-------+--------+---------------+---------+---------+-------
| id | select_type | table | type   | possible_keys | key     | key_len | ref
+----+-------------+-------+--------+---------------+---------+---------+-------
|  1 | SIMPLE      | a     | ALL    | NULL          | NULL    | NULL    | NULL
|  1 | SIMPLE      | cc    | const  | PRIMARY       | PRIMARY | 4       | const
|  1 | SIMPLE      | u     | eq_ref | PRIMARY       | PRIMARY | 4       | database
|  1 | SIMPLE      | g     | eq_ref | PRIMARY       | PRIMARY | 1       | database
+----+-------------+-------+--------+---------------+---------+---------+-------

Answer 1

据我所知，使用索引、提示或查询本身的重组无法合理解决此问题。

之所以慢，是因为它需要 2M 行的文件排序，这实际上需要很长时间。如果您放大订单，它指定为 ORDER BY a.title, a.created DESC。问题是对多于 1 列进行排序并具有 DESC 部分的组合。 Mysql 不支持降序索引(CREATE INDEX statement 中支持关键字 DESC，但仅供将来使用)。

建议的解决方法是创建一个额外的列“reverse_created”，该列会以您的查询可以使用 ORDER BY a.title, a.reverse_created 的方式自动填充。所以你用 max_time - created_time 填充它。然后在该组合上创建一个索引，并(如果需要)将该索引指定为提示。

有几篇关于这个主题的非常好的博客文章，通过示例更好地解释了这一点:

• http://www.mysqlperformanceblog.com/2006/05/09/descending-indexing-and-loose-index-scan/
• http://www.mysqlperformanceblog.com/2007/02/16/using-index-for-order-by-vs-restricting-number-of-rows/

-Update- 您应该能够通过从查询中的 order by 中删除“DESC”部分来对此进行快速测试。结果在功能上是错误的，但它应该使用您现有的索引(否则力应该起作用)。