Tenho a seguinte tabela e índice criados:

CREATE TABLE cdc_auth_user
(
  cdc_auth_user_id bigint NOT NULL DEFAULT nextval('cdc_auth_user_id_seq'::regclass),
  cdc_timestamp timestamp without time zone DEFAULT ('now'::text)::timestamp without time zone,
  cdc_operation text,
  id integer,
  username character varying(30)
);

CREATE INDEX idx_cdc_auth_user_cdc_timestamp
          ON cdc_auth_user
       USING btree (cdc_timestamp);

No entanto, quando executo uma seleção usando o campo de carimbo de data / hora, o índice está sendo ignorado e minha consulta leva quase 10 segundos para retornar:

EXPLAIN SELECT *
          FROM cdc_auth_user
         WHERE cdc_timestamp BETWEEN '1900/02/24 12:12:34.818'
                             AND '2012/02/24 12:17:45.963';


Seq Scan on cdc_auth_user  (cost=0.00..1089.05 rows=30003 width=126)
  Filter: ((cdc_timestamp >= '1900-02-24 12:12:34.818'::timestamp without time zone) AND (cdc_timestamp <= '2012-02-24 12:17:45.963'::timestamp without time zone))