Why historically do people use 255 not 256 for dat

Question

You often see database fields set to have a magnitude of 255 characters, what is the traditional / historic reason why? I assume it's something to do with paging / memory limits, and performance but the distinction between 255 and 256 has always confused me.

varchar(255)

Considering this is a capacity or magnitude, not an indexer, why is 255 preferred over 256? Is a byte reserved for some purpose (terminator or null or something)?

Presumably varchar(0) is a nonsense (has zero capacity)? In which case 2^8 of space should be 256 surely?

Are there other magnitudes that provide performance benefits? For example is varchar(512) less performant than varchar(511) or varchar(510)?

Is this value the same for all relations databases, old and new?

disclaimer - I'm a developer not a DBA, I use field sizes and types that suit my business logic where that is known, but I'd like to know the historic reason for this preference, even if it's no longer relevant (but even more if it still is relevant).

Edit:

Thanks for the answers, there seems to be some concensus that a byte is used to store size, but this doesn't settle the matter definitively in my mind.

If the meta data (string length) is stored in the same contiguous memory/disk, it makes some sense. 1 byte of metadata and 255 bytes of string data, would suit each other very nicely, and fit into 256 contiguous bytes of storage, which presumably is neat and tidy.

But...If the metadata (string length) is stored separately from the actual string data (in a master table perhaps), then to constrain the length of string's data by one byte, just because it's easier to store only a 1 byte integer of metadata seems a bit odd.

In both cases, it would seem to be a subtlety that probably depends on the DB implementation. The practice of using 255 seems pretty widespread, so someone somewhere must have argued a good case for it in the beginning, can anyone remember what that case was/is? Programmers won't adopt any new practice without a reason, and this must have been new once.

Answer 1

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

Answer 2

8 bits unsigned = 256 bytes

255 characters + byte 0 for length

Answer 3

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).

Answer 4

<<

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).

Answer 5

I think this might answer your question. Looks like it was the max limit of varchar in earlier systems. I took it off another stackoverflow question.

It's hard to know what the longest postal address is, of course, which is why many people choose a long VARCHAR that is certainly longer than any address. And 255 is customary because it may have been the maximum length of a VARCHAR in some databases in the dawn of time (as well as PostgreSQL until more recently).

Are there disadvantages to using a generic varchar(255) for all text-based fields?

Answer 6

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.