query not using index for descending records?

Hi All,

I have this table;

id - Serial
datetime - timestamp without timezone

with the index as

index idx_trafficlogs_datetime_id on trafficlogs using btree
(datetime,id);

When performing the following query:

explain select datetime,id from trafficlogs order by datetime,id limit
20;
                                                    QUERY PLAN
------------------------------------------------------------------------
--------------------------------------------Limit  (cost=0.00..2.31 rows=20 width=12)  ->  Index Scan using
idx_trafficlogs_datetime_idon trafficlogs 
(cost=0.00..1057.89 rows=9172 width=12)
(2 rows)

however, I am wanting to return the last 20 records. I have been using:

explain select datetime,id from trafficlogs order by datetime,id desc
limit 20;
                                  QUERY PLAN
------------------------------------------------------------------------
---------Limit  (cost=926.38..926.43 rows=20 width=12)  ->  Sort  (cost=926.38..949.31 rows=9172 width=12)        Sort
Key:datetime, id        ->  Seq Scan on trafficlogs  (cost=0.00..322.72 rows=9172 
width=12)


as you can see, a sequential scan is performed.

How do I get pg to use an index scan for this query. The table in a
production environment grows by approx 150,000 records daily, hence long
term performance is a major factor here - for each additional day of
data, the above query takes an additional 6-8 secs to run.

Tia,

Darren