mysql的explain命令可以分析sql的性能，其中有一項是key_len（索引的長度）的統計。本文將分析mysql explain中key_len的計算方法。
1、創建測試表及數據
| 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的字段類型是varchar(20)，字符編碼是utf8，一個字符占用3個字節，那么key_len應該是 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為63，多出了3。
name字段是允許NULL，把name改為NOT NULL再測試
| 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 |+----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+ |
現在key_len為62，比剛才少了1，但還是多了2。可以確定，字段為NULL會多占用一個字節。
name字段類型為varchar，屬于變長字段，把varchar改為char再測試
| 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 |+----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+ |
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