JDBC - update count

I have a relation with snapshots of rows from another relation. I want to
update the original relation to new values only if it is still equal to its
snapshot. For that, I send an update in a PreparedStatement, along these
lines:

UPDATE rel
SET col1=?, col2=?, col3=?, col4=?
FROM snapshot
WHERE snapshot.id = ?
    AND rel.col1 = snapshot.col1
    AND rel.col2 = snapshot.col2
    AND rel.col3 = snapshot.col3
    AND rel.col4 = snapshot.col4

Well, I assume that if the row in rel is different from the snapshot, I
will get a row count of zero returned from executeUpdate(). Instead, it
returns 1 - whether or not the update has actually affected any row.

I checked - the original row is not updated. No other has the new values,
so no other has been affected. And yet, it returns 1 for both a successful
and an unsuccessful update.

Is this my problem (misinterpretation of the executeUpdate API)? Or maybe
it's a bug in the JDBC driver? Or maybe it's a PostgreSQL bug?

Any alternative to doing the above? I'd really hate to break it into two
statements like:

SELECT rel.oid
FROM snapshot, rel
WHERE .....

and then

UPDATE rel
SET .....
WHERE oid = ?

Because I don't want to use oids if I can avoid it (there's talk about
their removal, and they are too system-specific anyway), and I'm not even
sure that this form of update won't give me the same result.

Herouth