255是mySQL4和更早版本中的varchar限制。

255字符+空终止符= 256

或者1字节长度的描述符给出0-255个字符的可能范围

255是可以存储在单字节无符号整数(假设8位字节)中的最大数值——因此，出于某种目的存储字符串长度的应用程序更喜欢255而不是256，因为这意味着它们只需要为“size”变量分配1个字节。

It used to be that all strings required a NUL terminator, or "backslash-zero". Updated databases don't have that. It was "255 characters of text" with a "\0" added automatically at the end so the system knew where the string ended. If you said VARCHAR(256), it would end up being 257 and then you'd be in the next register for one character. Wasteful. That's why everything was VARCHAR(255) and VARCHAR(31). Out of habit the 255 seems to have stuck around but the 31's became 32's and the 511's became 512's. That part is weird. It's hard to make myself write VARCHAR(256).

255是8位整数的最大值:11111111 = 255。

是否存在其他提供性能好处的级别?例如，varchar(512)的性能是否低于varchar(511)或varchar(510)?

回顾一下位/字节存储的基本原理，存储256以下的整数需要一个字节，存储256到65536之间的整数需要两个字节。 因此，它需要相同的空间(两个字节)来存储511或512或65535.... 因此，对于varchar(512)或varchar(511)，上面讨论中提到的this参数显然是N/A。

MySQL手册:

数据类型: VARCHAR (M), VARBINARY (M) 储存要求: 如果列值需要0 - 255字节，则为L + 1字节;如果值可能需要超过255字节，则为L + 2字节

理解并做出选择。