Re: Libpq: PQftype, PQfsize

>From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]
>> Thank you. In this case (substring) there is no much to
>predict, just
>> a simple calculation, but I understand that it is a part of
>larger and
>> more complicated functionality. I tried to find a workaround
>with a type cast:
>> select substr(fc,1,2)::varchar(2) from test Now the type returned is
>> varchar, but the size is still -1. I think that it is not a correct
>> return: the size is specified explicitly in the query and could be
>> used by PQfsize.
>
>Oh ... actually the problem there is that you have the wrong
>idea about what PQfsize means. What that returns is
>pg_type.typlen for the data type, which is always going to be
>-1 for a varlena type like varchar.
>
>The thing that you need to look at if you want to see
>information like the max length of a varchar is typmod
>(PQfmod). The typmod generally has some funny
>datatype-specific encoding; for varchar and char it works like this:
> -1: max length unknown or unspecified
> n>0: max length is n-4 characters

Thank you very much Tom. PQfmode returns the correct value when using a type
cast, so it solves my current problem.
Perhaps you will implement the exact column size for querries with character
functions somwhere in the future. It is a nice feature, which is implemented
by Oracle or MS SQL Server. Do not know about MySQL.

Regards,
Bozena Potempa