Seq Scan

Hi,
I'm having some problems in performance in a simple select count(id)
from.... I have 700 000 records in one table, and when I do:

# explain select (id) from table_name;
-[ RECORD 1 ]----------------------------------------------------------------
QUERY PLAN | Seq Scan on table_name  (cost=0.00..8601.30 rows=266730 width=4)

I had created an index for id(btree), but still shows "Seq Scan".
What I'm doing wrong?

Thanks,
Tyler