Question

mysql> explain select a.id,a.title from users c
    ->                                  straight_join iask a on c.id=a.uid
    ->                                         straight_join ianswer b on a.id=b.iaskid
    ->
    ->                                  where (c.last_check is null or b.created>c.last_check) and c.id in (1,2)
    ->                                  group by a.id;
+----+-------------+-------+-------+---------------------------+------------+---------+----------+------+----------------------------------------------+
| id | select_type | table | type  | possible_keys             | key        | key_len | ref      | rows | Extra                                        |
+----+-------------+-------+-------+---------------------------+------------+---------+----------+------+----------------------------------------------+
|  1 | SIMPLE      | c     | range | PRIMARY,i_users_lastcheck | PRIMARY    | 4       | NULL     |    2 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | a     | ref   | PRIMARY,i_iask_uid        | i_iask_uid | 4       | bbs.c.id |    2 |                                              |
|  1 | SIMPLE      | b     | ALL   | i_ianswer_iaskid          | NULL       | NULL    | NULL     |   17 | Using where; Using join buffer               |
+----+-------------+-------+-------+---------------------------+------------+---------+----------+------+----------------------------------------------+
3 rows in set (0.00 sec)

将上面的“in（1,2）”改为“= 1”将使用索引：

mysql> explain select a.id,a.title from iask a
    ->                                          join ianswer b on a.id=b.iaskid
    ->                                          join users c on c.id=a.uid
    ->                                  where (c.last_check is null or b.created>c.last_check) and c.id=1
    ->                                  group by a.id;
+----+-------------+-------+-------+---------------------------+------------------+---------+----------+------+---------------------------------+
| id | select_type | table | type  | possible_keys             | key              | key_len | ref      | rows | Extra                           |
+----+-------------+-------+-------+---------------------------+------------------+---------+----------+------+---------------------------------+
|  1 | SIMPLE      | c     | const | PRIMARY,i_users_lastcheck | PRIMARY          | 4       | const    |    1 | Using temporary; Using filesort |
|  1 | SIMPLE      | a     | ref   | PRIMARY,i_iask_uid        | i_iask_uid       | 4       | const    |    1 | Using where                     |
|  1 | SIMPLE      | b     | ref   | i_ianswer_iaskid          | i_ianswer_iaskid | 4       | bbs.a.id |    8 | Using where                     |
+----+-------------+-------+-------+---------------------------+------------------+---------+----------+------+---------------------------------+
3 rows in set (0.00 sec)

表结构如下：

mysql> show create table users;
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Table | Create Table
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| users | CREATE TABLE `users` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `email` varchar(128) NOT NULL,
  `password` varchar(32) NOT NULL,
  `screen_name` varchar(64) DEFAULT NULL,
  `reputation` int(10) unsigned NOT NULL DEFAULT '0',
  `imtype` varchar(1) DEFAULT '0' COMMENT '0--email,1--gtalk,2--msn',
  `last_check` datetime DEFAULT NULL,
  `robotno` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `u_users_email` (`email`),
  UNIQUE KEY `u_users_screen_name` (`screen_name`),
  KEY `i_users_lastcheck` (`last_check`),
  KEY `i_users_imtype_robotno` (`imtype`,`robotno`)
) ENGINE=InnoDB AUTO_INCREMENT=31 DEFAULT CHARSET=utf8 |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1 row in set (0.02 sec)

mysql> show create table iask;
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Table | Create Table
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| iask  | CREATE TABLE `iask` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `uid` int(10) unsigned NOT NULL,
  `title` varchar(250) NOT NULL,
  `body` text,
  `tags` varchar(100) NOT NULL,
  `views` int(10) unsigned DEFAULT '0',
  `votes` int(10) unsigned DEFAULT '0',
  `answer_id` int(10) unsigned DEFAULT NULL,
  `created` datetime NOT NULL,
  `keywords` text,
  PRIMARY KEY (`id`),
  KEY `i_iask_uid` (`uid`),
  FULLTEXT KEY `keywords` (`keywords`)
) ENGINE=MyISAM AUTO_INCREMENT=3 DEFAULT CHARSET=utf8 |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1 row in set (0.00 sec)

mysql> show create table ianswer;
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                                                                                                                                                                                                 |
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ianswer | CREATE TABLE `ianswer` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `iaskid` int(10) unsigned NOT NULL,
  `uid` int(10) unsigned DEFAULT NULL,
  `body` text,
  `votes` int(10) unsigned DEFAULT '0',
  `created` datetime NOT NULL,
  `anonymous` tinyint(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `i_ianswer_iaskid` (`iaskid`)
) ENGINE=InnoDB AUTO_INCREMENT=20 DEFAULT CHARSET=utf8 |
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql>

Answer 1

如果你问如何使第一个查询使用像第二个查询那样的索引，我怀疑这是因为你的表中没有足够的行来证明两个索引查找的合理性。全表扫描的行计数为17行，因此它认为只需旋转行寻找两个ID就行得更快。如果您担心，可以尝试填充表格以查看它是否仍然选择全表扫描。

如何在MYSQL中优化此查询？

1 个答案: