Indexes and querys

Hello list,

I have a table with an index in a foreing key field (int4), if I do
select from pedido where prvdfk=3, explain says is using seq scan to
access it. I noticed all my querys runs in that way, except in the
querys where the PK field (unique type index) is included in the where
clause.

Is this the rigth way that postgresql works ?, is there something I
should check in my configuration?

According to select version() this is the my one:
PostgreSQL 7.3.4 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.2
20020903 (Red Hat Linux 8.0 3.2-7)

Thanks in advance

--
Josué Maldonado.