Re: CHAR(N) storage requirement

David Garamond wrote:
> Peter Eisentraut wrote:
> >>Is it the 4+N (aka. same as VARCHAR(n)) or is it N? Sorry, it was
> >>100% not clear for me after reading the docs, though the docs imply
> >>the first: "The storage requirement for data of these types is 4
> >>bytes plus the actual string, and in case of character plus the
> >>padding."
> >
> > Storing varchar(n) takes 4 bytes plus as many bytes as are required to
> > store the actual string. This may be more or less then "n".
> >
> > Storing char(n) takes 4 bytes plus as many bytes are are required to
> > store the actual string, plus n - length(value) bytes for padding
> > spaces. This is at least "n" bytes.
>
> I see. Then there is no real benefits at all in using CHAR(N) in
> PostgreSQL, is that right? I wonder why there is no storage saving done
> by PostgreSQL for CHAR compared to VARCHAR (since the string length of
> CHAR field will be the same for all rows)... Or perhaps I can use array
> or CREATE TYPE to achieve this (that is, minimizing storage to only "n"
> bytes to store CHAR(n)). Assuming no multibyte/Unicode, only US-ASCII
> values.

The reason we store the length on disk for CHAR() is that the routines
to handle variable-length data types are used by all variable-length
data types, not just CHAR(), and there isn't an easy way to pass that
information around. The only way we could do it would be to pull the
row from disk, add needed lengths so they can be passed around to the
inernal routines, then strip them when writing --- it didn't seem worth
it.

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