reading table metadata from pl/pgsql in 8.2 under FreeBSD

Hi -

My apologies in advance if this is in the archives/documentation. I may not
have used the correct search terms in trying to find the answer.

I've looked at the system tables, particularly pg_tables, pg_class, and
pg_attribute. In oder to access the attributes for a particular table using
the pg_attribute table, I need the oid for the table, specifically the
pg_class.oid.

Looking at some tables, I believe that relfilenode is the the OID of the
table. The corresponding field (i.e., foreign key) in pg_attribute is
attrelid. So far so good. My problem is that in the pg_class table I don't
know how to determine which entries are tables. And in the pg_attribute
table I don't know how to determine which of the corresponding rows are
attributes. (My current rule of thumb to identify an attribute is that in
pg_attribute, the defined columns have attnum > 0; all others have attnum
<0. )

Thanks for any advice/suggestions.

david