以下查询正在使用连接缓冲区，我想知道是否有人可以向我解释为什么会这样。只是想更多地了解 mysql 和索引。

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                                  |
+----+-------------+-------------+--------+---------------------------------------+-----------------+---------+--------------------------------------+------+----------+----------------------------------------------+

事件表:

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

事件主题表:

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

位置表:

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 

最佳答案

正如它在 'http://dev.mysql.com/doc/refman/5.1/en/explain-output.html 中所说的那样':“来自早期连接的表被部分读入连接缓冲区，然后使用缓冲区中的行来执行与当前表的连接。”

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加入缓冲区的 event_topic 内容。

使用缓冲区是一件好事；您可能在 EXPLAIN 输出的第一行注意到了不需要的“Using temporary; Using filesort”，这可能来自 GROUP BY，在这种情况下可能是不可避免的。

顺便问一下，您会遇到 city_name 上的“UNIQUE”约束的问题吗？我在考虑斯普林菲尔德(新泽西州有两个)、华盛顿、格林维尔等地。

关于mysql - 为什么 MySQL 查询使用连接缓冲区？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/19318082/