Query on postgresql 7.4.2 not using index

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