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?| +----+-------------+--------+------+---------------+------+---------+-------+------+-----------------------+
改為定長字段后，key_len為60，與預測的一致。
總結：使用變長字段需要額外增加2個字節，使用NULL需要額外增加1個字節，因此對于是索引的字段，最好使用定長和NOT NULL定義，提高性能。
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