Question

仍然在优化MySQL LEFT JOIN时遇到问题。该查询需要0.13秒才能完成而不是0.00（下一个（简化版））。

我想为该查询达到0.00左右。

我尝试过全面创建索引和组合索引。变化不大。基本上只要在EXPLAIN中有FILESORT，就会很慢。我不知道该怎么做...在表格之间创建一个索引？它甚至存在吗？

谢谢。

罪魁祸首：

SELECT 
  SQL_NO_CACHE p.id 
FROM 1_posts p 
  INNER JOIN 1_topics t 
    ON (p.cid = t.cid && p.container = t.id) 
WHERE 
  t.cid = 1010699 
ORDER BY 
  p.id DESC 
LIMIT 1;

EXPLAIN输出：

+----+-------------+-------+------+-------------------+-------+---------+---------------------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys     | key   | key_len | ref                 | rows | Extra                                        |
+----+-------------+-------+------+-------------------+-------+---------+---------------------+------+----------------------------------------------+
|  1 | SIMPLE      | t     | ref  | PRIMARY,cid,cid_2 | cid   | 4       | const               |  216 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | p     | ref  | PRIMARY,cid,cid_2 | cid_2 | 8       | const,forumdb.t.id |   12 |                                              |
+----+-------------+-------+------+-------------------+-------+---------+---------------------+------+----------------------------------------------+

现在，同样简化的查询工作正常（使用索引等。唯一的区别是括号之间）：

SELECT 
  SQL_NO_CACHE p.id 
FROM 
  1_posts p 
  INNER JOIN 1_topics t 
    ON (p.cid = t.cid) 
WHERE 
  t.cid = 1010699 
ORDER BY 
  p.id DESC 
LIMIT 1;

说明：

+----+-------------+-------+-------+-------------------+---------+---------+-------+-------+--------------------------+
| id | select_type | table | type  | possible_keys     | key     | key_len | ref   | rows  | Extra                    |
+----+-------------+-------+-------+-------------------+---------+---------+-------+-------+--------------------------+
|  1 | SIMPLE      | p     | range | PRIMARY,cid,cid_2 | PRIMARY | 4       | NULL  | 31720 | Using where; Using index |
|  1 | SIMPLE      | t     | ref   | PRIMARY,cid,cid_2 | cid_2   | 4       | const |   194 | Using index              |
+----+-------------+-------+-------+-------------------+---------+---------+-------+-------+--------------------------+

表：

CREATE TABLE `1_posts` (
  `cid` int(20) unsigned NOT NULL DEFAULT '0',
  `id` int(20) unsigned NOT NULL AUTO_INCREMENT,
  `container` int(20) unsigned NOT NULL DEFAULT '0',
  `creator` int(20) unsigned NOT NULL DEFAULT '0',
  `ref` int(20) unsigned DEFAULT NULL,
  `timestamp` int(20) unsigned NOT NULL DEFAULT '0',
  `posticon` tinyint(11) DEFAULT NULL,
  `last_edited_ts` int(10) unsigned DEFAULT NULL,
  `last_edited_by` int(20) unsigned DEFAULT NULL,
  `signature` varchar(250) DEFAULT NULL,
  `client_ip` int(10) unsigned NOT NULL DEFAULT '0',
  `data_format` tinyint(20) unsigned DEFAULT NULL,
  `use_bbcode` tinyint(3) unsigned NOT NULL DEFAULT '1',
  `use_smileys` tinyint(3) unsigned NOT NULL DEFAULT '1',
  `topic_hash` int(10) unsigned NOT NULL DEFAULT '0',
  `del_ts` int(10) unsigned NOT NULL DEFAULT '0',
  `del_reason` tinyint(4) NOT NULL DEFAULT '0',
  PRIMARY KEY (`cid`,`id`),
  UNIQUE KEY `cid` (`cid`,`topic_hash`,`container`,`id`,`del_ts`),
  KEY `cid_2` (`cid`,`container`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8


CREATE TABLE `1_topics` (
  `cid` int(10) unsigned NOT NULL DEFAULT '0',
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `container` int(20) NOT NULL DEFAULT '0',
  `name` varchar(128) NOT NULL DEFAULT '',
  `creator` int(20) unsigned NOT NULL DEFAULT '0',
  `last_modified` int(20) unsigned NOT NULL DEFAULT '0',
  `views` int(11) NOT NULL DEFAULT '0',
  `closed` tinyint(3) unsigned NOT NULL DEFAULT '0',
  `sticky` tinyint(3) unsigned NOT NULL DEFAULT '0',
  `last_post_id` int(20) unsigned DEFAULT NULL,
  `num_posts` int(10) unsigned DEFAULT NULL,
  `lp_ts` int(20) unsigned NOT NULL DEFAULT '0',
  `posticon` smallint(5) unsigned DEFAULT NULL,
  `hidden` tinyint(3) unsigned NOT NULL DEFAULT '0',
  `topic_change_ts` int(10) unsigned NOT NULL DEFAULT '0',
  `topic_hash` int(10) unsigned NOT NULL DEFAULT '0',
  `forum_hash` int(10) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`cid`,`id`),
  KEY `container` (`container`),
  KEY `last_modified` (`last_modified`),
  KEY `sticky` (`sticky`),
  KEY `topic_hash` (`topic_hash`),
  KEY `forum_hash` (`forum_hash`),
  KEY `cid` (`cid`,`id`),
  KEY `cid_2` (`cid`),
  FULLTEXT KEY `name` (`name`)
) ENGINE=MyISAM AUTO_INCREMENT=211963 DEFAULT CHARSET=latin1

这是戈登指数补充后的EXPLAIN输出：

+----+-------------+-------+------+-------------------------------+-------+---------+---------------------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys                 | key   | key_len | ref                 | rows | Extra                                        |
+----+-------------+-------+------+-------------------------------+-------+---------+---------------------+------+----------------------------------------------+
|  1 | SIMPLE      | t     | ref  | PRIMARY,cid,cid_2             | cid   | 4       | const               |  212 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | p     | ref  | PRIMARY,cid,cid_2,cid_3,cid_4 | cid_3 | 8       | const,forumdb.t.id |   11 | Using index                                  |
+----+-------------+-------+------+-------------------------------+-------+---------+---------------------+------+----------------------------------------------+

Answer 1

此版本使用正确的索引：

SELECT SQL_NO_CACHE p.id
FROM 1_posts p INNER JOIN
     1_topics t
     ON (p.cid = t.cid)
WHERE t.cid = 1010699
ORDER BY p.id DESC LIMIT 1;

此版本不会：

SELECT SQL_NO_CACHE p.id
FROM 1_posts p INNER JOIN
     1_topics t
     ON (p.cid = t.cid && p.container = t.id);
WHERE t.cid = 1010699
ORDER BY p.id DESC
LIMIT 1;

首先，MySQL可以首先使用l_posts(cid, id)上的索引作为where子句（cid列是索引中的第一个），然后是连接（同一列）。然后它可以使用相同的索引进行排序 - id是索引中的下一列。（顺便说一下，这是使用MySQL优化器的一个功能，它将=条件中的where条件从t传播到p。）

对于第二个，MySQL可以使用l_posts(cid, container)和where的{{1}}索引。但是，相同的索引不能用于排序。引擎决定文件排序比尝试合并两个不同的索引更好。

要使第二个版本使用索引，请在join上定义一个。

Answer 2

尝试为第二个表添加WHERE条件：

SELECT
SQL_NO_CACHE p.id
FROM 
  1_posts p 
  INNER JOIN 1_topics t 
    ON (p.cid = t.cid) 
WHERE 
  t.cid = 1010699 AND p.id > 0
ORDER BY 
p.id DESC 
LIMIT 1;

这个解决方案对我有用。

MySQL连接优化 - 绕过filesort

2 个答案: