按日期desc排序时，“使用临时”会降低查询速度

2018-07-03 14:19:54

我有一个日志条目表，以及约100个可能的日志代码的说明表：

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行的10k）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，当添加ORDER BY log_entries.logentry_id DESC ，这当然是必要的，它会减慢到0.6秒。可能是因为“使用临时”在log_codes表上使用？删除索引实际上会使查询执行得更快，但仍然很慢（0.3秒）。

EXPLAIN输出不带ORDER BY的查询：

+----+-------------+-------------+------+----------------------------+------------------+---------+--------------------------+------+-------------+
| 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                     |
+----+-------------+-------------+------+----------------------------+------------------+---------+--------------------------+------+---------------------------------+

有关如何让此查询更快执行的任何提示？我不明白为什么应该需要“使用临时”，因为在提取和排序相应的日志条目之前应该选择日志代码？

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 |                                 |
+----+-------------+-------------+------+-------------------------+------------------+---------+-------------------+------+---------------------------------+

UPDATE @RolandoMySQLDBA ：

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                     |
+----+-------------+-------------+------+------------------+------------------+---------+------+-------+---------------------------------+

用你的索引，不要订购：

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       |
+----+-------------+-------------+-------+-----------------------+-----------------------+---------+------+-------+--------------------------------+

使用索引，ORDER BY日期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，这里和类似问题中的大多数问题都来自于误解MySQL（和其他数据库）如何使用索引进行排序。答案是：MySQL不使用索引进行排序，它只能以索引顺序或相反方向读取数据。如果您碰巧想按照当前使用的索引顺序对数据进行排序 - 那么您很幸运，否则结果将被排序（因此EXPLAIN中的filesort）

这是整个结果的顺序主要取决于哪个表是第一个连接。如果你看看你的EXPLAIN，你会看到连接从'log_codes'表开始（因为它更小）。

基本上，您需要的是'log_entries'上的组合索引（partner_id，date），'log_codes'的覆盖组合索引（log_code，category_overview，log_desc），将'INNER JOIN'更改为'STRAIGHT_JOIN'以强制连接顺序，并按照'date'DESC命令（这个指数还会覆盖太多）。

UPD1 ：很抱歉，我错误地键入了第一个表的索引：它应该是(partner_id, log_code, date) 。

但是当我尝试对另一个表中的列进行排序时，我仍然很难理解为什么MySQL选择在log_codes表上使用“临时”（以及100x查询时间）？

只要你同意它的顺序，MySQL就可以直接输出数据，或者把数据放到临时表中，然后应用排序和输出。当您通过连接中任何非第一个表中的字段进行排序时，MySQL必须对数据进行排序（而不仅仅是按索引顺序输出），并将它需要的数据排序为临时表。

但随着我对数据集的深入研究，它会变得更慢（LIMIT 50000,25秒）。你知道为什么吗？

要输出行50000,25，MySQL无论如何都需要获取第一个50000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光盘查找log_code值。覆盖索引应该快得多，因为所有数据都可以从索引中获取。

UPD2 ：试图强制索引：

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;

你将需要两件事

对查询进行折衷

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);

创建索引来支持子查询和减少表访问

在创建这些索引之前，运行它们

SELECT COUNT(1) rowcount,partner_id FROM log_entries GROUP BY partner_id;
SELECT COUNT(1) rowcount,category_overview FROM log_codes GROUP BY category_overview;

如果所有可能的partner_id值中的计数都不超过log_entries表的5％，请创建此索引

ALTER TABLE log_entries ADD INDEX (partner_id,log_code,date);

如果所有可能的category_overview值中的计数都不超过log_codes表的5％，请创建此索引

ALTER TABLE log_codes ADD INDEX (category_overview,log_code,log_desc);

试一试！！！

请尝试包含LIMIT 0,25重构查询

SELECT log_entries.date, log_codes.log_desc FROM 
(
    SELECT A.log_code FROM 
    (SELECT log_code FROM log_entries WHERE partner_id = 1) A INNER JOIN
    (SELECT log_code FROM log_codes WHERE category_overview = 1) B USING (log_code)
    LIMIT 0,25
) log_code_keys
INNER JOIN log_entries USING (log_code)
INNER JOIN log_code USING (log_code);

我首先反转IX_partner_code和IX_overview_code索引中的列。这应该使它们更适合同时支持JOIN和WHERE子句。

...
KEY `IX_code_partner` (`log_code`,`partner_id`)
...
KEY `IX_code_overview` (`log_code`,`category_overview`),
...

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