MySQL中采用类型varchar(20)和varchar(255)对性能上的影响

CREATE DATABASE TestDataBase
USE TestDataBase
CREATE TABLE ABC (
  `id` int(11) DEFAULT NULL,
  `name` varchar(20) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
ALTER  TABLE  `ABC`  ADD  INDEX `nameIndex` (`name`)
explain select name from ABC
alter table ABC  modify name varchar(255)
explain select name from ABC

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)固定字段且允许NULL:10*(Character Set：utf8=3,gbk=2,latin1=1)