more performance on some aggregates

I've created a table, and index.
Table = test
+----------------------------+----------------------------+-------+
| Field | Type | Length|
+----------------------------+----------------------------+-------+
| a | int4 | 4 |
| c | varchar | 40 |
+----------------------------+----------------------------+-------+
Index: _test_a
if i need the max or min value of column a i can get with
SELECT max(test.a); query.
If i take look on the query tree:
Aggregate (cost=43.00 rows=1000 width=4)
-> Seq Scan on test (cost=43.00 rows=1000 width=4)
...
Ok, i understand: all type of the aggregates work on same way, but...
have i any chance to get extrem values, without seq scan?

Thanx in advice

--
nek;(