以下查询正在使用联接缓冲区，我想知道是否有人可以向我解释为什么会这样。只是想了解更多关于mysql和索引的理解。
mysql> EXPLAIN SELECT events.event_topic_id, event_topic_name, event_topic_image, event_type_name,city_name FROM events
-> JOIN event_topic ON event_topic.event_topic_id=events.event_topic_id
-> JOIN event_type ON event_type.event_type_id = event_topic.event_type_id
-> JOIN locations ON locations.location_id=events.location_id
-> JOIN city ON city.city_id=locations.city_id
-> WHERE event_date > NOW()
-> GROUP BY events.event_topic_id, city.city_id;
+----+-------------+-------------+--------+---------------------------------------+-----------------+---------+--------------------------------------+------+----------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------------+--------+---------------------------------------+-----------------+---------+--------------------------------------+------+----------+----------------------------------------------+
| 1 | SIMPLE | city | index | PRIMARY | city_name | 52 | NULL | 6 | 100.00 | Using index; Using temporary; Using filesort |
| 1 | SIMPLE | locations | ref | PRIMARY,city_id | city_id | 1 | PremiumCONNECT.city.city_id | 1 | 100.00 | Using index |
| 1 | SIMPLE | events | ref | location_id,event_topic_id,event_date | location_id | 2 | PremiumCONNECT.locations.location_id | 3 | 100.00 | Using where |
| 1 | SIMPLE | event_type | index | PRIMARY | event_type_name | 52 | NULL | 2 | 100.00 | Using index; Using join buffer |
| 1 | SIMPLE | event_topic | eq_ref | PRIMARY,event_type_id | PRIMARY | 1 | PremiumCONNECT.events.event_topic_id | 1 | 100.00 | Using where |
+----+-------------+-------------+--------+---------------------------------------+-----------------+---------+--------------------------------------+------+----------+----------------------------------------------+
活动表：
CREATE TABLE `events` (
`event_id` smallint(8) unsigned NOT NULL AUTO_INCREMENT,
`location_id` smallint(3) unsigned NOT NULL,
`event_date` datetime NOT NULL,
`event_topic_id` tinyint(3) unsigned NOT NULL,
PRIMARY KEY (`event_id`),
KEY `location_id` (`location_id`),
KEY `event_topic_id` (`event_topic_id`),
KEY `event_date` (`event_date`),
CONSTRAINT `events_ibfk_2` FOREIGN KEY (`event_topic_id`) REFERENCES `event_topic` (`event_topic_id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `events_ibfk_3` FOREIGN KEY (`location_id`) REFERENCES `locations` (`location_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=91 DEFAULT CHARSET=latin1
活动主题表：
CREATE TABLE `event_topic` (
`event_topic_id` tinyint(3) unsigned NOT NULL AUTO_INCREMENT,
`event_topic_name` varchar(100) DEFAULT NULL,
`event_topic_description` text NOT NULL,
`event_topic_cost` decimal(7,2) DEFAULT NULL,
`event_type_id` tinyint(3) unsigned NOT NULL,
`event_topic_clickthrough` tinytext,
`event_topic_length` varchar(6) NOT NULL,
`event_topic_image` varchar(41) DEFAULT NULL,
`event_topic_image_md5` char(32) NOT NULL,
PRIMARY KEY (`event_topic_id`),
KEY `event_type_id` (`event_type_id`),
KEY `topic_image_sha1` (`event_topic_image_md5`),
CONSTRAINT `event_topic_ibfk_1` FOREIGN KEY (`event_type_id`) REFERENCES `event_type` (`event_type_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=14 DEFAULT CHARSET=latin1
事件类型表：
CREATE TABLE `event_type` (
`event_type_id` tinyint(3) unsigned NOT NULL AUTO_INCREMENT,
`event_type_name` varchar(50) NOT NULL,
`conf_email` text,
PRIMARY KEY (`event_type_id`),
KEY `event_type_name` (`event_type_name`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1
地点信息表：
CREATE TABLE `locations` (
`location_id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`location_name` varchar(50) NOT NULL,
`location_address` tinytext NOT NULL,
`location_capacity` smallint(6) NOT NULL,
`city_id` tinyint(3) unsigned NOT NULL,
`gps_coords` varchar(30) DEFAULT NULL,
PRIMARY KEY (`location_id`),
KEY `city_id` (`city_id`),
CONSTRAINT `locations_ibfk_1` FOREIGN KEY (`city_id`) REFERENCES `city` (`city_id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=13 DEFAULT CHARSET=latin1
城市表：
CREATE TABLE `city` (
`city_id` tinyint(3) unsigned NOT NULL AUTO_INCREMENT,
`city_name` varchar(50) NOT NULL,
PRIMARY KEY (`city_id`),
UNIQUE KEY `city_name` (`city_name`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=latin1
答案 0 :(得分：9)
正如它在'http://dev.mysql.com/doc/refman/5.1/en/explain-output.html'中所说：“来自早期联接的表被分成几部分读入连接缓冲区，然后从缓冲区中使用它们的行来执行与当前表的连接。” p>
因此，在您的情况下，您已经加入了event_topic，因此优化器能够使用来自连接缓冲区的event_topic内容。
使用缓冲区是一件好事;您可能已经注意到EXPLAIN输出的第一行上不合需要的“使用临时;使用filesort”，这可能来自GROUP BY，在这种情况下可能是不可避免的。
顺便说一句，你会遇到city_name上“UNIQUE”约束的问题吗？我在考虑斯普林菲尔德（新泽西州的两个），华盛顿，格林维尔等。
答案 1 :(得分：1)
尝试使用：
"STRAIGHT_JOIN" and "FORCE INDEX":
EXPLAIN SELECT events.event_topic_id, event_topic_name, event_topic_image, event_type_name,city_name FROM events
-> straight_join event_topic force index(primary) ON event_topic.event_topic_id=events.event_topic_id
-> straight_join event_type force index(primary) ON event_type.event_type_id = event_topic.event_type_id
-> straight_join locations force index(primary) ON locations.location_id=events.location_id
-> straight_join city force index(primary) ON city.city_id=locations.city_id
-> WHERE event_date > NOW()
-> GROUP BY events.event_topic_id, city.city_id;