1、mysql执行查询计划，key_len表示索引使用的字节数，这个字节数和三个条件有关。
mysql> create table t1(v1 char(10));
Query OK, 0 rows affected

mysql> create index index_v1 on t1(v1);
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0

2、是否允许为null，允许为null 会多一个字节，标识取值是否为NULL。如下：
mysql> alter table t1 modify column v1 char(10) not null;
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | t1 | index | NULL | index_v1 | 30 | NULL | 1 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
1 row in set

mysql> alter table t1 modify column v1 char(10) null;
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | t1 | index | NULL | index_v1 | 31 | NULL | 1 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
1 row in set
3、是否为变长，变长多出两个字节表示长度，如下：
mysql> alter table t1 modify column v1 varchar(10) null;
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | t1 | index | NULL | index_v1 | 33 | NULL | 1 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
1 row in set

mysql> alter table t1 modify column v1 char(10) null;
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | t1 | index | NULL | index_v1 | 31 | NULL | 1 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
1 row in set
4、字符类型，varchar(n) 中的n是字符个数，不是占用多少个字节的内存。对于不同的编码，同样一个字符占用的内存不一样。latin1 一个字符占用1个字节，gb2312 一个字符占用2个字节，utf8 一个字符占用3个字节。
mysql> alter table t1 modify column v1 char(10) charset latin1;
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | t1 | index | NULL | index_v1 | 11 | NULL | 1 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
1 row in set

mysql> alter table t1 modify column v1 char(10) charset gb2312;
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | t1 | index | NULL | index_v1 | 21 | NULL | 1 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
1 row in set

mysql> alter table t1 modify column v1 char(10) charset utf8;
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| 1 | SIMPLE | t1 | index | NULL | index_v1 | 31 | NULL | 1 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
1 row in set