如何在MYSQL中優化這個查詢？

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) 

變化 「在（1,2）」 到上述 「= 1」 將使其所有使用指數：

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) 

表結構如下：

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

如果詢問如何使第一個查詢像第二個查詢一樣使用索引，我懷疑這是因爲表中沒有足夠的行來證明兩個索引查找。全表掃描的行數爲17行，所以它認爲只需旋轉行尋找兩個ID就可以更快。如果您擔心，您可以嘗試填充表格以查看它是否仍然選擇全表掃描。