explain的结果中关于key_len的一个疑问

Question

[email protected]_monitor>explain select * from test where value1='';
+----+-------------+-------+------+-------------------+-------------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys     | key               | key_len | ref   | rows | Extra       |
+----+-------------+-------+------+-------------------+-------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | test  | ref  | idx_value1_value2 | idx_value1_value2 | 13      | const |    1 | Using where | 
+----+-------------+-------+------+-------------------+-------------------+---------+-------+------+-------------+
1 row in set (0.00 sec)[email protected]_monitor>desc test;
+--------+------------+------+-----+---------+----------------+
| Field  | Type       | Null | Key | Default | Extra          |
+--------+------------+------+-----+---------+----------------+
| id     | tinyint(4) | NO   | PRI | NULL    | auto_increment | 
| value  | tinyint(4) | YES  |     | NULL    |                | 
| value1 | char(4)    | YES  | MUL | NULL    |                | 
| value2 | char(4)    | YES  |     | NULL    |                | 
+--------+------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)这个key_len为啥是13呢？show create table test;
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                                          |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| test  | CREATE TABLE `test` (
  `id` tinyint(4) NOT NULL auto_increment,
  `value` tinyint(4) default NULL,
  `value1` char(4) collate utf8_bin default NULL,
  `value2` char(4) collate utf8_bin default NULL,
  PRIMARY KEY  (`id`),
  KEY `idx_value1_value2` (`value1`,`value2`)
) ENGINE=InnoDB AUTO_INCREMENT=128 DEFAULT CHARSET=utf8 COLLATE=utf8_bin | 
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

Answer 1

 KEY `idx_value1_value2` (`value1`,`value2`)| value1 | char(4) 
| value2 | char(4) 这两个字段就8个长度了吧，索引中还会包括5字节长的定位信息。

Answer 2

我的char_set是utf-8，按理说是1-4个字节，应该是4*4*2=32另外我还试验了一下了单个tinyint字段的索引，观察length是1，另外关于“定位”信息文档上有描述吗？找了半天没有任何资料

Answer 3

这个length是字节长度  
value1 | char(4)  utf8 4*3=12字节+1字节null
When a nullable column is indexed, it requires an extra 
byte per entry

Answer 4

试了不同的create，确实如此。之所以value2没有起作用(ref=const)，是因为你的select用不到value2，假如
explain select * from test where value1='' and value2='';那么value2就起作用了(ref=const,const)，长度也要计算value2，比如，我设置value1 char(5) not null,
则
explain select * from test where value1=''; length = 15 = 5 * 3
而
explain select * from test where value1='' and value2=''; length = 28 = 5 * 3 + 4 * 3 + 1