how to optimize this query in 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)

change the above "in (1,2)" into "=1" will make it all using index:

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)

table structure as follows:

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

If you are asking how to make the first query use an index like the second query does, I suspect it is because there aren't enough rows in your table to justify two index lookups. The row count for a full table scan is 17 rows, so it thinks it is faster just to spin across the rows looking for two ids. If you are concerned, you could try populating the table to see if it still chooses a full table scan.