浅谈mysql explain中key_len的计算方法

mysql的explain命令可以分析sql的性能,其中有一项是key_len(索引的长度)的统计。本文将分析mysql explain中key_len的计算方法。

1、创建测试表及数据

CREATE TABLE `member` (
 `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
 `name` varchar(20) DEFAULT NULL,
 `age` tinyint(3) unsigned DEFAULT NULL,
 PRIMARY KEY (`id`),
 KEY `name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `member` (`id`, `name`, `age`) VALUES (NULL, 'fdipzone', '18'), (NULL, 'jim', '19'), (NULL, 'tom', '19');

2、查看explain

name的字段类型是varchar(20),字符编码是utf8,一个字符占用3个字节,那么key_len应该是 20*3=60

mysql> explain select * from `member` where name='fdipzone';
+----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref  | rows | Extra         |
+----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+
| 1 | SIMPLE   | member | ref | name     | name | 63   | const |  1 | Using index condition |
+----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+

explain的key_len为63,多出了3

name字段是允许NULL,把name改为NOT NULL再测试

ALTER TABLE `member` CHANGE `name` `name` VARCHAR(20) NOT NULL;

mysql> explain select * from `member` where name='fdipzone';
+----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref  | rows | Extra         |
+----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+
| 1 | SIMPLE   | member | ref | name     | name | 62   | const |  1 | Using index condition |
+----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+

现在key_len为62,比刚才少了1,但还是多了2。可以确定,字段为NULL会多占用一个字节。

name字段类型为varchar,属于变长字段,把varchar改为char再测试

ALTER TABLE `member` CHANGE `name` `name` CHAR(20) NOT NULL;

mysql> explain select * from `member` where name='fdipzone';
+----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref  | rows | Extra         |
+----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+
| 1 | SIMPLE   | member | ref | name     | name | 60   | const |  1 | Using index condition |
+----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+

改为定长字段后,key_len为60,与预测的一致。

总结:使用变长字段需要额外增加2个字节,使用NULL需要额外增加1个字节,因此对于是索引的字段,最好使用定长和NOT NULL定义,提高性能。

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