按日期降序排序时，“使用临时”会减慢查询速度

我有一个日志条目表，以及大约 100 个可能的日志代码的描述表：

CREATE TABLE `log_entries` (
  `logentry_id` int(11) NOT NULL AUTO_INCREMENT,
  `date` datetime NOT NULL,
  `partner_id` smallint(4) NOT NULL,
  `log_code` smallint(4) NOT NULL,
  PRIMARY KEY (`logentry_id`),
  KEY `IX_code` (`log_code`),
  KEY `IX_partner_code` (`partner_id`,`log_code`)
) ENGINE=MyISAM ;

CREATE TABLE IF NOT EXISTS `log_codes` (
  `log_code` smallint(4) NOT NULL DEFAULT '0',
  `log_desc` varchar(255) DEFAULT NULL,
  `category_overview` tinyint(1) NOT NULL DEFAULT '0',
  `category_error` tinyint(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`log_code`),
  KEY `IX_overview_code` (`category_overview`,`log_code`),
  KEY `IX_error_code` (`category_error`,`log_code`)
) ENGINE=MyISAM ;

以下查询（匹配 20k 行中的 10k 行）在 0.0034 秒内执行（使用LIMIT 0,20):

SELECT log_entries.date, log_codes.log_desc FROM log_entries 
INNER JOIN log_codes ON log_codes.log_code = log_entries.log_code 
WHERE log_entries.partner_id = 1 AND log_codes.category_overview = 1;

但是添加时ORDER BY log_entries.logentry_id DESC，这当然是必要的，它减慢到0.6秒。可能是因为 log_codes 表上使用了“使用临时”？删除索引实际上使查询执行得更快，但仍然很慢（0.3 秒）。

EXPLAIN 不带 ORDER BY 的查询输出：

+----+-------------+-------------+------+----------------------------+------------------+---------+--------------------------+------+-------------+
| id | select_type | table       | type | possible_keys              | key              | key_len | ref                      | rows | Extra       |
+----+-------------+-------------+------+----------------------------+------------------+---------+--------------------------+------+-------------+
|  1 | SIMPLE      | log_codes   | ref  | PRIMARY,IX_overview_code   | IX_overview_code | 1       | const                    |   56 |             |
|  1 | SIMPLE      | log_entries | ref  | IX_code,IX_partner_code    | IX_partner_code  | 7       | const,log_codes.log_code |   25 | Using where |
+----+-------------+-------------+------+----------------------------+------------------+---------+--------------------------+------+-------------+
  

并包括 ORDER BY：

+----+-------------+-------------+------+----------------------------+------------------+---------+--------------------------+------+---------------------------------+
| id | select_type | table       | type | possible_keys              | key              | key_len | ref                      | rows | Extra                           |
+----+-------------+-------------+------+----------------------------+------------------+---------+--------------------------+------+---------------------------------+
|  1 | SIMPLE      | log_codes   | ref  | PRIMARY,IX_overview_code   | IX_overview_code | 1       | const                    |   56 | Using temporary; Using filesort |
|  1 | SIMPLE      | log_entries | ref  | IX_code,IX_partner_code    | IX_partner_code  | 7       | const,log_codes.log_code |   25 | Using where                     |
+----+-------------+-------------+------+----------------------------+------------------+---------+--------------------------+------+---------------------------------+
  

关于如何让这个查询执行得更快有什么提示吗？我不明白为什么需要“使用临时”，因为应该在获取和排序适当的日志条目之前选择日志代码？

更新@Eugen Rieck:

SELECT log_entries.date, lc.log_desc FROM log_entries INNER JOIN (SELECT log_desc, log_code FROM log_codes WHERE category_overview = 1) AS lc ON lc.log_code = log_entries.log_code WHERE log_entries.partner_id = 1 ORDER BY log_entries.logentry_id;
+----+-------------+-------------+------+-------------------------+------------------+---------+-------------------+------+---------------------------------+
| id | select_type | table       | type | possible_keys           | key              | key_len | ref               | rows | Extra                           |
+----+-------------+-------------+------+-------------------------+------------------+---------+-------------------+------+---------------------------------+
|  1 | PRIMARY     | <derived2>  | ALL  | NULL                    | NULL             | NULL    | NULL              |   57 | Using temporary; Using filesort |
|  1 | PRIMARY     | log_entries | ref  | IX_code,IX_partner_code | IX_partner_code  | 7       | const,lc.log_code |   25 | Using where                     |
|  2 | DERIVED     | log_codes   | ref  | IX_overview_code        | IX_overview_code | 1       |                   |   56 |                                 |
+----+-------------+-------------+------+-------------------------+------------------+---------+-------------------+------+---------------------------------+
  

更新@RolandoMySQLDBA:

使用我的原始索引，ORDER BY date DESC：

SELECT log_entries.date, log_codes.log_desc FROM (SELECT log_code,date FROM log_entries WHERE partner_id = 1) log_entries INNER JOIN (SELECT log_code,log_desc FROM log_codes WHERE category_overview = 1) log_codes USING (log_code) ORDER BY log_entries.date DESC;
+----+-------------+-------------+------+------------------+------------------+---------+------+-------+---------------------------------+
| id | select_type | table       | type | possible_keys    | key              | key_len | ref  | rows  | Extra                           |
+----+-------------+-------------+------+------------------+------------------+---------+------+-------+---------------------------------+
|  1 | PRIMARY     | <derived3>  | ALL  | NULL             | NULL             | NULL    | NULL |    57 | Using temporary; Using filesort |
|  1 | PRIMARY     | <derived2>  | ALL  | NULL             | NULL             | NULL    | NULL | 21937 | Using where; Using join buffer  |
|  3 | DERIVED     | log_codes   | ref  | IX_overview_code | IX_overview_code | 1       |      |    56 |                                 |
|  2 | DERIVED     | log_entries | ALL  | IX_partner_code  | NULL             | NULL    | NULL | 22787 | Using where                     |
+----+-------------+-------------+------+------------------+------------------+---------+------+-------+---------------------------------+
  

使用您的索引，无需排序：

SELECT log_entries.date, log_codes.log_desc FROM (SELECT log_code,date FROM log_entries WHERE partner_id = 1) log_entries INNER JOIN (SELECT log_code,log_desc FROM log_codes WHERE category_overview = 1) log_codes USING (log_code);
+----+-------------+-------------+-------+-----------------------+-----------------------+---------+------+-------+--------------------------------+
| id | select_type | table       | type  | possible_keys         | key                   | key_len | ref  | rows  | Extra                          |
+----+-------------+-------------+-------+-----------------------+-----------------------+---------+------+-------+--------------------------------+
|  1 | PRIMARY     | <derived3>  | ALL   | NULL                  | NULL                  | NULL    | NULL |    57 |                                |
|  1 | PRIMARY     | <derived2>  | ALL   | NULL                  | NULL                  | NULL    | NULL | 21937 | Using where; Using join buffer |
|  3 | DERIVED     | log_codes   | index | IX_overview_code_desc | IX_overview_code_desc | 771     | NULL |    80 | Using where; Using index       |
|  2 | DERIVED     | log_entries | index | IX_partner_code_date  | IX_partner_code_date  | 15      | NULL | 22787 | Using where; Using index       |
+----+-------------+-------------+-------+-----------------------+-----------------------+---------+------+-------+--------------------------------+
  

使用您的索引，ORDER BY date DESC：

SELECT log_entries.date, log_codes.log_desc FROM (SELECT log_code,date FROM log_entries WHERE partner_id = 1) log_entries INNER JOIN (SELECT log_code,log_desc FROM log_codes WHERE category_overview = 1) log_codes USING (log_code) ORDER BY log_entries.date DESC;
+----+-------------+-------------+-------+-----------------------+-----------------------+---------+------+-------+---------------------------------+
| id | select_type | table       | type  | possible_keys         | key                   | key_len | ref  | rows  | Extra                           |
+----+-------------+-------------+-------+-----------------------+-----------------------+---------+------+-------+---------------------------------+
|  1 | PRIMARY     | <derived3>  | ALL   | NULL                  | NULL                  | NULL    | NULL |    57 | Using temporary; Using filesort |
|  1 | PRIMARY     | <derived2>  | ALL   | NULL                  | NULL                  | NULL    | NULL | 21937 | Using where; Using join buffer  |
|  3 | DERIVED     | log_codes   | index | IX_overview_code_desc | IX_overview_code_desc | 771     | NULL |    80 | Using where; Using index        |
|  2 | DERIVED     | log_entries | index | IX_partner_code_date  | IX_partner_code_date  | 15      | NULL | 22787 | Using where; Using index        |
+----+-------------+-------------+-------+-----------------------+-----------------------+---------+------+-------+---------------------------------+
  

更新@Joe Stefanelli:

SELECT log_entries.date, log_codes.log_desc FROM log_entries INNER JOIN log_codes ON log_codes.log_code = log_entries.log_code WHERE log_entries.partner_id = 1 AND log_codes.category_overview = 1 ORDER BY date DESC;
+----+-------------+-------------+------+--------------------------+-----------------+---------+--------------------------+------+----------------------------------------------+
| id | select_type | table       | type | possible_keys            | key             | key_len | ref                      | rows | Extra                                        |
+----+-------------+-------------+------+--------------------------+-----------------+---------+--------------------------+------+----------------------------------------------+
|  1 | SIMPLE      | log_codes   | ALL  | PRIMARY,IX_code_overview | NULL            | NULL    | NULL                     |   80 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | log_entries | ref  | IX_code,IX_code_partner  | IX_code_partner | 7       | log_codes.log_code,const |   25 | Using where                                  |
+----+-------------+-------------+------+--------------------------+-----------------+---------+--------------------------+------+----------------------------------------------+
  

我认为，这里的大多数问题以及类似的问题都来自于对 MySQL（和其他数据库）如何使用索引进行排序的误解。答案是：MySQL不使用索引进行排序，它只是可以按照索引的顺序或相反的方向读取数据。如果您碰巧希望按照当前使用的索引的顺序对数据进行排序 - 您很幸运，否则结果将被排序（因此 EXPLAIN 中的文件排序）

也就是说，整个结果的顺序主要取决于哪个表是连接中的第一个表。如果您查看 EXPLAIN，您会发现连接从“log_codes”表开始（因为它小得多）。

基本上，您需要的是“log_entries”上的复合索引（partner_id，date），“log_codes”的覆盖复合索引（log_code，category_overview，log_desc），将“INNER JOIN”更改为“STRAIGHT_JOIN”以强制连接顺序，并按“日期”DESC 排序（幸运的是，该索引也将被覆盖）。

UPD1：抱歉，我输错了第一个表的索引：它应该是(partner_id, log_code, date).

但我仍然很难理解当我尝试对另一个表中的列进行排序时，为什么 MySQL 选择在 log_codes 表上“使用临时”（和 100 倍的查询时间）？

MySQL可以直接输出数据，只要你同意它获取数据的顺序，或者将数据放在临时表中，然后排序并输出。当您在连接中按任何非第一个表中的字段排序时，MySQL 必须对数据进行排序（不仅仅是按索引的顺序输出），并且为了对数据进行排序，它需要一个临时表。

但当我进一步了解数据集时，它会变慢（LIMIT 50000,25 为 6 秒）。你知道为什么吗？

要输出第 50000,25 行，MySQL 无论如何都需要获取前 50000 行并跳过它们。由于我错过了索引中的一列，MySQL 不仅扫描了索引，而且还为每个项目进行了额外的磁盘查找log_code价值。使用覆盖索引应该会更快，因为所有数据都可以从索引中获取。

UPD2：尝试强制索引：

SELECT log_entries.date, log_codes.log_desc
FROM log_entries FORCE INDEX (IX_partner_code_date)
STRAIGHT_JOIN log_codes
  ON log_codes.log_code = log_entries.log_code
WHERE log_entries.partner_id = 1
  AND log_codes.category_overview = 1
ORDER BY log_entries.date DESC;