index on id and created_at

Hi,

If I have a table like...

CREATE TABLE apa
(
 objectid                   SERIAL PRIMARY KEY NOT NULL
,created_at                 TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL
,somedata                   text
)
WITHOUT OIDS;

...where if rowX har higher objectid than rowY, it is implied that rowX
has a later created_at then rowY.

select
    objectid
from
    apa
where
    created_at > now() - interval '1 day'
order by
    objectid desc;

In this select, it would have been nice to dodge the full table scan
without adding an index to created_at. Is this possible somehow?

Best regards,
Marcus