optimizer question

Hi,

I have a table that contains almost 8 milion rows. The primary key is a
sequence, so the index should have a good distribution. Why does the
optimizer refuse to use the index for getting the maximum value?
(even after a vacuum analyze of the table)

radius=# explain select max(radiuspk) from radius ;
NOTICE: QUERY PLAN:

Aggregate (cost=257484.70..257484.70 rows=1 width=8)
-> Seq Scan on radius (cost=0.00..237616.76 rows=7947176 width=8)

Table and key info:

Did not find any relation named "radius_pk".
radius=# \d radius
Table "radius"
Attribute | Type | Modifier
---------------------+--------------------------+---------------------------
sessionid | character varying(30) | not null
username | character varying(30) | not null
nas_ip | character varying(50) | not null
logfileid | integer |
login_ip_host | character varying(50) | not null
framed_ip_address | character varying(50) |
file_timestamp | timestamp with time zone | not null
corrected_timestamp | timestamp with time zone | not null
acct_status_type | smallint | not null
bytesin | bigint |
bytesout | bigint |
handled | boolean | not null default 'f'
sessionhandled | boolean | not null default 'f'
radiuspk | bigint | not null default nextval
('radiuspk_seq'::text)
Indices: pk_radius,
radius_us

radius=# \d pk_radius
Index "pk_radius"
Attribute | Type
-----------+--------
radiuspk | bigint
unique btree (primary key)