Bug?

When I execute the following query:

"select p.id, s.name from phys p, phys_spec s where p.r_phys_spec = s.id and
p.r_phys_spec > 0"

.... it returns nothing. However if I add " AND p.id=585" to the end of that
query, it returns _one result_.

How is that possible? Add an extra condition and get back more results? The
second query logically must return a set equal to or smaller than the second.

Am I missing something? What does a person generally do in a situation like
this?

Background : v 6.4.2, downloaded and compiled like 4 days ago, on Red Hat 5.2
w/ gcc 2.7.2.

TIA
tom