Mysql索引长度计算

(root:hostname:Fri May 29 14:10:50 2015)[liangxl]> show create table liang_2;
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------
| Table   | Create Table                                                                                                                                                
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------
| liang_2 | CREATE TABLE `liang_2` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `uid` int(11) DEFAULT NULL,
  `name` varchar(20) DEFAULT NULL,
  `email` varchar(20) DEFAULT NULL,
  `addr` varchar(20) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `ind_liang_2_name` (`name`(5)),
  KEY `ind_liang_2_email` (`email`)
) ENGINE=InnoDB AUTO_INCREMENT=35 DEFAULT CHARSET=utf8 |
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------
1 row in set (0.00 sec)
(root:hostname:Fri May 29 14:10:56 2015)[liangxl]> create index ind_liang_2_addr on liang_2(addr);
Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0
(root:hostname:Fri May 29 14:11:13 2015)[liangxl]> explain select * from liang_2 where addr='asdsdadasdads';
+----+-------------+---------+------+------------------+------------------+---------+-------+------+-------------+
| id | select_type | table   | type | possible_keys    | key              | key_len | ref   | rows | Extra       |
+----+-------------+---------+------+------------------+------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | liang_2 | ref  | ind_liang_2_addr | ind_liang_2_addr | 62      | const |    4 | Using where |
+----+-------------+---------+------+------------------+------------------+---------+-------+------+-------------+
1 row in set (0.00 sec)
(root:hostname:Fri May 29 14:11:32 2015)[liangxl]> explain select * from liang_2 where email='asas@ss';     
+----+-------------+---------+------+-------------------+-------------------+---------+-------+------+-------------+
| id | select_type | table   | type | possible_keys     | key               | key_len | ref   | rows | Extra       |
+----+-------------+---------+------+-------------------+-------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | liang_2 | ref  | ind_liang_2_email | ind_liang_2_email | 63      | const |    4 | Using where |
+----+-------------+---------+------+-------------------+-------------------+---------+-------+------+-------------+
1 row in set (0.01 sec)
(root:hostname:Fri May 29 14:11:51 2015)[liangxl]> explain select * from liang_2 where name='aaaab';   
+----+-------------+---------+------+------------------+------------------+---------+-------+------+-------------+
| id | select_type | table   | type | possible_keys    | key              | key_len | ref   | rows | Extra       |
+----+-------------+---------+------+------------------+------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | liang_2 | ref  | ind_liang_2_name | ind_liang_2_name | 18      | const |    1 | Using where |
+----+-------------+---------+------+------------------+------------------+---------+-------+------+-------------+
1 row in set (0.00 sec)
(root:hostname:Fri May 29 14:12:25 2015)[liangxl]> \s;
Server characterset:    utf8
Db     characterset:      utf8
Client characterset:      utf8
Conn.  characterset:    utf8