背景：
当用Explain查看SQL的执行计划时，里面有列显示了 key_len 的值，根据这个值可以判断索引的长度，在组合索引里面可以更清楚的了解到了哪部分字段使用到了索引。
环境：
CREATE TABLE `tmp_0612` ( `id` int(11) NOT NULL, `name` varchar(10) DEFAULT NULL, `age` int(11) DEFAULT NULL, `address` varchar(100) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8
插入数据：
insert into tmp_0612 values(1,'a',11,'hz'),(2,'b',22,'gz'),(3,'c',33,'aa');
创建索引：
alter table tmp_0612 add index idx_name(name); alter table tmp_0612 add index idx_age(age);
测试：
explain select * from tmp_0612 where name ='a'; +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | tmp_0612 | ref | idx_name | idx_name | 33 | const | 1 | Using index condition | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+
从上面的执行计划可知，索引的长度是33。比预想的30（10*3(utf8)）要高出3字节，为什么呢？进一步测试：
修改name 成 not null
alter table tmp_0612 modify name varchar(10) not null;
再看执行计划：
explain select * from tmp_0612 where name ='a'; +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | tmp_0612 | ref | idx_name | idx_name | 32 | const | 1 | Using index condition | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+
发现上面的执行计划和第一次的有区别(key_len)，经过多次测试，发现字段允许NULL的会多出一个字节。想到了之前的一篇文章，NULL是需要一个标志位的，占用1个字符。那还有2个字节怎么算？这里想到的是会不会和 多字节字符集 相关？那改字符集试试：
alter table tmp_0612 convert to charset latin1; explain select * from tmp_0612 where name ='a'; +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | tmp_0612 | ref | idx_name | idx_name | 12 | const | 1 | Using index condition | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+
发现还是多了2个字节，看来和多字节字符集没有关系了。那会不会和 变长字段 有关系？再试试：
alter table tmp_0612 convert to charset utf8; alter table tmp_0612 modify name char(10) not null; explain select * from tmp_0612 where name ='a'; +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | tmp_0612 | ref | idx_name | idx_name | 30 | const | 1 | Using index condition | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+
和预料的一样了，是30=10*3。到这里相信大家都已经很清楚了，要是还比较模糊就看反推到33字节。
改成允许NULL，应该会变成31。
alter table tmp_0612 modify name char(10); explain select * from tmp_0612 where name ='a'; +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | tmp_0612 | ref | idx_name | idx_name | 31 | const | 1 | Using index condition | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+
改成变长字段类型，应该会变成33。
alter table tmp_0612 modify name varchar(10); explain select * from tmp_0612 where name ='a'; +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | tmp_0612 | ref | idx_name | idx_name | 33 | const | 1 | Using index condition | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+
改成单字节字符集，还是还是需要额外的3字节（1：null ；变长字段：2），和字符集无关。
alter table tmp_0612 convert to charset latin1; explain select * from tmp_0612 where name ='a'; +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | tmp_0612 | ref | idx_name | idx_name | 13 | const | 1 | Using index condition | +----+-------------+----------+------+---------------+----------+---------+-------+------+-----------------------+
反推上去都和预测的一样。
其他测试：
explain select * from tmp_0612 where age = 11; +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | tmp_0612 | ref | idx_age | idx_age | 5 | const | 1 | NULL | +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+ alter table tmp_0612 modify age int not null; explain select * from tmp_0612 where age = 11; +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | tmp_0612 | ref | idx_age | idx_age | 4 | const | 1 | NULL | +----+-------------+----------+------+---------------+---------+---------+-------+------+-------+
int 是占4个字符的，上面key_len的也是符合预期。关于组合索引的，可以自己去测试玩。
总结：
变长字段需要额外的2个字节，固定长度字段不需要额外的字节。而null都需要1个字节的额外空间，所以以前有个说法：索引字段最好不要为NULL，因为NULL让统计更加复杂，并且需要额外一个字节的存储空间。
key_len的长度计算公式：
varchr(10)变长字段且允许NULL : 10*(Character Set：utf8=3,gbk=2,latin1=1)+1(NULL)+2(变长字段)
varchr(10)变长字段且不允许NULL : 10*(Character Set：utf8=3,gbk=2,latin1=1)+2(变长字段)
char(10)固定字段且允许NULL : 10*(Character Set：utf8=3,gbk=2,latin1=1)+1(NULL)
char(10)固定字段且不允许NULL : 10*(Character Set：utf8=3,gbk=2,latin1=1)