latin1 字符集, 65532
utf8 字符集, 21844
utf8mb4 字符集, 16128
也就是说如果设定为 utf8mb4 字符集, varchar 最多能存 16128 个字符, 不论是中文还是英文.
(65535-1-2)/1=65532
(65535-1-2)/3=21844
(65535-1-2)/4=16383 !=16128, 不满足这个"公式"
官方文档: https://dev.mysql.com/doc/refman/5.7/en/char.html
The effective maximum length of a VARCHAR is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used.