mysql key_len各种字段类型计算公式

char和varchar类型key_len计算公式:varchr(N)变长字段且允许NULL   =  N * ( character set：utf8=3,gbk=2,latin1=1)+1(NULL)+2(变长字段)varchr(N)变长字段且不允许NULL =  N * ( character set：utf8=3,gbk=2,latin1=1)+2(变长字段)
char(N)固定字段且允许NULL     =  N * ( character set：utf8=3,gbk=2,latin1=1)+1(NULL)char(N)固定字段且允许NULL     =  N * ( character set：utf8=3,gbk=2,latin1=1) 数值数据的key_len计算公式：TINYINT允许NULL = 1 + 1(NULL)TINYINT不允许NULL = 1SMALLINT允许为NULL = 2+1(NULL)SMALLINT不允许为NULL = 2INT允许为NULL = 4+1(NULL)INT不允许为NULL = 4日期时间型的key_len计算：（针对mysql5.5及之前版本）DATETIME允许为NULL =  8 + 1(NULL)DATETIME不允许为NULL = 8TIMESTAMP允许为NULL = 4 + 1(NULL)TIMESTAMP不允许为NULL = 4