Re: Libpq: PQftype, PQfsize

"Bozena Potempa" <Bozena(dot)Potempa(at)otc(dot)pl> writes:
> 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

regards, tom lane