时间：2021-02-09 14:30:57 | 栏目：Mysql | 点击：次
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。