Query on postgresql 7.4.2 not using index

[Arnau <arnaulist@xxxxxxxxxxxxxxxxxx>]

Hi all,

  I have the following running on postgresql version 7.4.2:

CREATE SEQUENCE agenda_user_group_id_seq
MINVALUE 1
MAXVALUE 9223372036854775807
CYCLE
INCREMENT 1
START 1;

CREATE TABLE AGENDA_USERS_GROUPS
(
 AGENDA_USER_GROUP_ID  INT8
                       CONSTRAINT pk_agndusrgrp_usergroup PRIMARY KEY
                       DEFAULT NEXTVAL('agenda_user_group_id_seq'),
 USER_ID               NUMERIC(10)
                       CONSTRAINT fk_agenda_uid  REFERENCES 
AGENDA_USERS (USER_ID)
                       ON DELETE CASCADE
                       NOT NULL,
 GROUP_ID              NUMERIC(10)
                       CONSTRAINT fk_agenda_gid  REFERENCES 
AGENDA_GROUPS (GROUP_ID)
                       ON DELETE CASCADE
                       NOT NULL,
 CREATION_DATE         DATE
                       DEFAULT CURRENT_DATE,
                       CONSTRAINT un_agndusrgrp_usergroup 
UNIQUE(USER_ID, GROUP_ID)
);

CREATE INDEX i_agnusrsgrs_userid ON AGENDA_USERS_GROUPS ( USER_ID );
CREATE INDEX i_agnusrsgrs_groupid ON AGENDA_USERS_GROUPS ( GROUP_ID );


When I execute:

EXPLAIN ANALYZE SELECT agenda_user_group_id FROM agenda_users_groups 
WHERE group_id = 9;

it does a sequential scan and doesn't use the index and I don't 
understand why, any idea? I have the same in postgresql 8.1 and it uses 
the index :-|

Thanks
--
Arnau