columns of views at all (not only oid, cmin etc. too).
    pgsql=> select cmin from pg_rules;
    ERROR:  system column cmin not available - pg_rules is a view
    pgsql=> select * from pg_rules where pg_rules.oid = pg_class.oid;
    ERROR:  system column oid not available - pg_rules is a view
    pgsql=>

Jan