comment optimiser cette requête dans MYSQL?

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) 

changement ci-dessus "dans (1,2)" dans "= 1" fera tout index à l'aide:

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) 

structure de la table

comme suit:

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

Si vous êtes En demandant comment faire pour que la première requête utilise un index comme le fait la deuxième requête, je suppose que c'est parce qu'il n'y a pas assez de lignes dans votre table pour justifier deux recherches d'index. Le nombre de lignes pour un scan de table complet est de 17 lignes, donc il pense qu'il est plus rapide de faire défiler les lignes à la recherche de deux identifiants. Si vous êtes concerné, vous pouvez essayer de remplir la table pour voir si elle choisit toujours une analyse de table complète.