If the column is empty, then applied to the data type uses 1 byte basis, such as int type, can not be empty key_len 4, 5 can be empty key_len 2. I ran a query in Mysql like below: EXPLAIN SELECT * FROM( SELECT * # Select Number 2 FROM post WHERE parentid = 13 ORDER BY time, id LIMIT 1, 10 ) post13_childs JOIN post post13_childs_childs ON post13_childs_childs.parentid = post13_childs.id 즉, dept_no 칼럼의 타입이 CHAR(4) 이기 때문에 프라이머리 키 앞쪽에서 12바이트만 유효하게 사용했다는 의미다. How it works. MySQLにて、SQLをexplainした時に表示される項目の大事なポイントをメモ。ちなみに、本ページに表示されているサンプルSQL用テーブル作成DDLはこちら。（必須ではないので閉じておく）ポイントは、サンプルテーブル

Because the @s string contains 2-byte characters, its length in character is 19, while its length in bytes is 38. It is possible to do this by hand , or to read EXPLAIN ’ s output directly , but it requires patience and expertise . pt-visual-explain reverse-engineers MySQL ’ s EXPLAIN output into a query execution plan, which it then formats as a left-deep tree – the same way the plan is represented inside MySQL.

First, we convert the MySQL String Length string into ucs2 character set, which is UCS-2 Unicode that holds 2-byte characters. The above query will force MySQL server to conduct a full table scan (start to finish) to retrieve the record that we are searching.

MySQL will first try to use an index, e.g. 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。 以上这篇浅谈mysql explain中key_len的计算方法就是小编分享给大家的全部内容了，希望能给大家一个参 … 그래서 key_len 칼럼의 값이 12로 표시된 것이다. 改为定长字段后，key_len为60，与预测的一致。. do a range scan on your table a using the search-key.It estimates to get 174 rows out of using that index, which is the number in rows.This step is not yet called filtering. In MySQL, EXPLAIN can be used in front of a query beginning with SELECT, INSERT, DELETE, REPLACE, and UPDATE.

To filter here means to apply a condition on a set of rows that were selected by a type-search as potential rows, and to only keep rows that fulfill the condition:. ; Second, we use the CHAR_LENGTH and LENGTH functions to get the length of the @s string in bytes and in characters.

explain五：possible_keys、key和key_len介绍 1. possible_keys：显示可能应用在这张表中的索引，一个或多个。查询涉及到的字段上若存在索引，则该索引奖杯列出，但不一定被查询实际使用。

Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. MySQL in key_len calculation rules In MySQL, key_len calculation rules are as follows: 1. Learn more Mysql Explain Select “operation”, “key_len” and “ref”