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