query don't optimize

hi folks
the query:

SELECT P.ACT_INCORPORADA, P.FECHA_INICIO,
P.OBSERVACIONES1
FROM PASEST AS P
WHERE
P.FECHA_INICIO between '1999-08-22
00:00:00.000000'::timestamp AND '1999-12-22
23:59:00.000000'::timestamp
ORDER BY P.ACT_INCORPORADA

make table scan , the planner don't optimize it, the
table definition is

CREATE TABLE pasest
(
act_principal char(24) NOT NULL,
fecha_inicio timestamp NOT NULL,
act_incorporada char(24) NOT NULL,
codigo_incorporado char(1) NOT NULL,
fecha_fin timestamp,
id_repart_origen int4 NOT NULL,
id_repart_destino int4 NOT NULL,
fojas numeric(4) NOT NULL,
recibo_suelto char(1),
ficha_tramite numeric(6),
numer_remito int4,
id_reparticion_u int4 NOT NULL,
observaciones1 varchar(250),
observaciones2 varchar(250),
cod_permanencia char(2),
estado_pase char(1),
paq_actua_anterior char(1) NOT NULL DEFAULT
''::bpchar,
actua_caratulacion char(1) NOT NULL DEFAULT
''::bpchar,
param_01 char(1) NOT NULL DEFAULT ''::bpchar,
param_02 char(1) NOT NULL DEFAULT ''::bpchar,
param_03 char(1) NOT NULL DEFAULT ''::bpchar,
fts_observaciones tsvector,
CONSTRAINT pk_pasest PRIMARY KEY (act_principal,
fecha_inicio, act_incorporada),
CONSTRAINT fk_permanencia FOREIGN KEY
(cod_permanencia)
REFERENCES permanet (codigo_permanencia) MATCH
SIMPLE
ON UPDATE RESTRICT ON DELETE RESTRICT,
CONSTRAINT fx_caratult FOREIGN KEY (act_principal)
REFERENCES caratult (actuacion_car) MATCH SIMPLE
ON UPDATE RESTRICT ON DELETE RESTRICT,
CONSTRAINT fx_reparticion_destino FOREIGN KEY
(id_repart_destino)
REFERENCES repartit (id_reparticion) MATCH
SIMPLE
ON UPDATE RESTRICT ON DELETE RESTRICT,
CONSTRAINT fx_reparticion_id_reparticion_u FOREIGN
KEY (id_reparticion_u)
REFERENCES repartit (id_reparticion) MATCH
SIMPLE
ON UPDATE RESTRICT ON DELETE RESTRICT,
CONSTRAINT fx_reparticion_origen FOREIGN KEY
(id_repart_origen)
REFERENCES repartit (id_reparticion) MATCH
SIMPLE
ON UPDATE RESTRICT ON DELETE RESTRICT
)
WITHOUT OIDS;
ALTER TABLE pasest OWNER TO postgres;

and have this index

CREATE INDEX ix9_pasest
ON pasest
USING btree
(fecha_inicio);

why planner don't optimize it ?
any ideas?

"PostgreSQL 7.4.3 on i386-pc-linux-gnu, compiled by
GCC 2.95.4"

Linux serverf 2.4.26-1-686-smp #1 SMP
4 gigabytes
hp proliant series

TIA
best regards
MDC

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