Is there a good reason I see VARCHAR 255 used so often as opposed to another length

Question

In multiple courses  books  and jobs  I have seen text fields defined as VARCHAR 255  as kind of the default for  shortish  text   Is there any good reason that a length of 255 is chosen so often  other than being a nice round number   Is it a holdout from some time in the past when there was a good reason  whether or not it applies today    I realize  of course  that a tighter limit would be more ideal  if you somehow know the maximum length of the string   But if you are using VARCHAR 255  that probably indicates that you don t know the max length  only that it is a  shortish  string     Note  I found this question  varchar 255  v tinyblob v tinytext   which says that VARCHAR n  requires n 1 bytes of storage for n lt  255  n 2 bytes of storage for n gt 255   Is this the only reason   That seems kind of arbitrary  since you would only be saving two bytes compared to VARCHAR 256   and you could just as easily save another two bytes by declaring it VARCHAR 253

User · Accepted Answer

Historically  255 characters has often been the maximum length of a VARCHAR in some DBMSes  and it sometimes still winds up being the effective maximum if you want to use UTF-8 and have the column indexed  because of index length limitations

User · Answer

In many applications  like MsOffice  until version 2000 or 2002   the maximum number of characters per cell was 255  Moving data from programs able of handling more than 255 characters per field to from those applications was a nightmare  Currently  the limit is less and less hindering

User · Answer

0000 0000 -  this is an 8-bit binary number  A digit represents a bit    You count like so    0000 0000    0   0000 0001    1   0000 0010    2    0000 0011    3   Each bit can be one of two values  on or off   The total highest number can be represented by multiplication    2   2   2   2   2   2   2   2 - 1   255   Or   2 8 - 1     We subtract one because the first number is 0     255 can hold quite a bit  no pun intended  of values     As we use more bits the max value goes up exponentially   Therefore for many purposes  adding more bits is overkill

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Another reason may be that in very old data access libraries on Windows such as RDO and ADO  COM version not ADO NET  you had to call a special method  GetChunk   to get data from a column with more than 255 chars  If you limited a varchar column to 255  this extra code wasn t necessary

User · Answer

I m going to answer the literal question   no  there isn t a good reason you see VARCHAR 255  used so often  there are indeed reasons  as discussed in the other answers  just not good ones    You won t find many examples of projects that have failed catastrophically because the architect chose VARCHAR 300  instead of VARCHAR 255    This would be an issue of near-total insignificance even if you were talking about CHAR instead of VARCHAR

User · Answer

255 is used because it s the largest number of characters that can be counted with an 8-bit number   It maximizes the use of the 8-bit count  without frivolously requiring another whole byte to count the characters above 255   When used this way  VarChar only uses the number of bytes   1 to store your text  so you might as well set it to 255  unless you want a hard limit  like 50  on the number of characters in the field

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An unsigned 1 byte number can contain the range  0-255  inclusive   So when you see 255  it is mostly because programmers think in base 10  get the joke       Actually  for a while  255 was the largest size you could give a VARCHAR in MySQL  and there are advantages to using VARCHAR over TEXT with indexing and other issues

User · Answer

Probably because both SQL Server and Sybase  to name two I am familiar with  used to have a 255 character maximum in the number of characters in a VARCHAR column   For SQL Server  this changed in version 7 in 1996 1997 or so    but old habits sometimes die hard

User · Answer

Note  I found this question  varchar 255  v tinyblob v tinytext   which says that VARCHAR n  requires n 1 bytes of storage for n lt  255  n 2 bytes of storage for n gt 255   Is this the only reason   That seems kind of arbitrary  since you would only be saving two bytes compared to VARCHAR 256   and you could just as easily save another two bytes by declaring it VARCHAR 253     No  you don t save two bytes by declaring 253  The implementation of the varchar is most likely a length counter and a variable length  nonterminated array  This means that if you store  hello  in a varchar 255  you will occupy 6 bytes  one byte for the length  the number 5  and 5 bytes for the five letters

User · Answer

When you say 2 8 you get 256  but the numbers in computers terms begins from the number 0  So  then you got the 255  you can probe it in a internet mask for the IP or in the IP itself   255 is the maximum value of a 8 bit integer   11111111   255  Does that help

[database] Is there a good reason I see VARCHAR(255) used so often (as opposed to another length)?

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