don't use index, why?

Hello,

I have some doubt about index.
Look the follow example:

naslog=# \d desconexao
Table "desconexao"
Attribute | Type | Modifier
----------------+-------------+-------------------------------------------------
id | integer | not null default
nextval('id_desconexao'::text)
time | timestamp |
client_user | varchar(20) |
client | varchar(40) |
ip_nas | inet |
ip_client_user | inet |
disconnect | smallint |
user_time | interval |
data_rate | integer |
called_number | varchar(14) |
calling_number | varchar(14) |
filtrado | boolean | default 'f'::bool
Indices: client_desconexao_idx,
desconexao_pkey,
filtro_idx,
time_idx

naslog=# \d time_idx
Index "time_idx"
Attribute | Type
-----------+-----------
time | timestamp
btree

naslog=# explain select * from desconexao where time = '2000-12-01';
NOTICE: QUERY PLAN:

Index Scan using time_idx on desconexao (cost=0.00..20.18 rows=5
width=103)

EXPLAIN

Every thing is perfect, but when i execute de follow query:

naslog=# explain select * from desconexao where time > '2000-12-01';
NOTICE: QUERY PLAN:

Seq Scan on desconexao (cost=0.00..19547.71 rows=657958 width=103)

EXPLAIN

Why postgresql don't use the time_idx index? I only changed the operator
"=" to ">" in
clause "where".

Thank you.
Regards
-- Marco Catunda