An explanation of the calculation method of key_len in mysql explain
Mysql's explain command can analyze the performance of sql, one of which is the statistics of key_len (the length of the index). This article will analyze the calculation method of key_len in mysql explain.
1. Create test table and data
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. View explain
## The field type of
varchar(20), the character encoding is utf8, one character occupies 3 bytes, then key_len should be 20*3= 60. The key_len of
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 | +----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+
63, which is 3.
The name field allows NULL, Change the name to NOT NULL and test again
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 | +----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+
62, 1 less than before, but still 2 more. It is certain that a NULL field will occupy one more byte.
The name field type is varchar, which is a variable-length field. Change varchar to char and test again
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 | +----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+
60, which is consistent with the prediction.
Summary: Using variable-length fields requires an additional 2 bytes, using NULL requires an additional 1 bytes, Therefore, for indexed fields, it is best to use fixed length and NOT NULL definitions to improve performance.
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