Why historically do people use 255 not 256 for database field magnitudes?

Question

With a maximum length of 255 characters, the DBMS can choose to use a single byte to indicate the length of the data in the field. If the limit were 256 or greater, two bytes would be needed.

A value of length zero is certainly valid for varchar data (unless constrained otherwise). Most systems treat such an empty string as distinct from NULL, but some systems (notably Oracle) treat an empty string identically to NULL. For systems where an empty string is not NULL, an additional bit somewhere in the row would be needed to indicate whether the value should be considered NULL or not.

As you note, this is a historical optimisation and is probably not relevant to most systems today.

255 was the varchar limit in mySQL4 and earlier.

Also 255 chars + Null terminator = 256

Or 1 byte length descriptor gives a possible range 0-255 chars

255 is the largest numerical value that can be stored in a single-byte unsigned integer (assuming 8-bit bytes) - hence, applications which store the length of a string for some purpose would prefer 255 over 256 because it means they only have to allocate 1 byte for the "size" variable.

From MySQL Manual:

Data Type :
VARCHAR(M), VARBINARY(M)

Storage Required:
L + 1 bytes if column values require 0 – 255 bytes, L + 2 bytes if values may require more than 255 bytes

Understand and make choice.

255 is the maximum value of a 8 bit integer : 11111111 = 255.

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Recollected the fundamentals of the bits/bytes storage, it requires one byte to store integers below 256 and two bytes for any integer between 256 and 65536. Hence, it requires same space (two bytes) to store 511 or 512 or for that matter 65535.... Thus it is clear that the this argument mentioned in the discussion above is N/A for varchar(512) or varchar(511).

A maximum length of 255 allows the database engine to use only 1 byte to store the length of each field. You are correct that 1 byte of space allows you to store 2^8=256 distinct values for the length of the string.

But if you allow the field to store zero-length text strings, you need to be able to store zero in the length. So you can allow 256 distinct length values, starting at zero: 0-255.