help with query!!!

hi all
i�m have one query (see below) and not is possible to
me optimize, so moving to postgres release 7.3.2 but
i�cant optimize the query an don't understood why

additional data.
the table shape
                Table "public.Transitos"    Column      |            Type             |
Modifiers
-----------------+-----------------------------+-----------codigoEstacion  | character(2)                | not
nullnumeroVia       | smallint                    | not
nullfechaHora       | timestamp(3) with time zone | not
nullmedioPago       | character varying(50)       | not
nulltipoTransito    | character(20)               | not
nullcategoria       | character(20)               | not
nullcontrolTransito | character varying(50)       | not
nullcontrolPago     | character varying(50)       | not
nulldescripcion     | character varying(150)      | not
null
Indexes: transitos_pkey primary key btree
("codigoEstacion", "numeroVia", "fechaHora",
"medioPago", "tipoTransito", categoria),        i_t2 btree ("codigoEstacion", "numeroVia",
"fechaHora", "medioPago", "tipoTransito", categoria),        i_transitos btree ("codigoEstacion",
"numeroVia", "fechaHora", "medioPago", "tipoTransito",
categoria),        it_3 btree ("codigoEstacion", "numeroVia",
"fechaHora", "tipoTransito", "medioPago", categoria)

the querie:

explain delete from "Transitos"
where "codigoEstacion"= '02' and
"numeroVia" = 1 and
"fechaHora" = '2003-0403 17:34:06.92'::timestamp and
"medioPago" = 'Efectivo' and
"tipoTransito"= 'Normal' and
categoria='01'


result:Seq Scan on "Transitos"  (cost=0.00..10612.03 rows=1
width=6)  Filter: (("codigoEstacion" = '02'::bpchar) AND
("numeroVia" = 1) AND ("fechaHora" = ('2003-04-03
17:34:06.92'::timestamp without time zone)::timestamp
with time zone) AND ("medioPago" 'Efectivo'::character varying) AND ("tipoTransito" 'Normal'::bpchar) AND (categoria =
'01'::bpchar))
(2 rows)




question: why them perform one seq scan ? if you see
have index covering where expression ?
any ideas?

TIA

PD: SORRY FOR MY ENGLISH IS NOT MY MOTHER LANGUAGE =;o)

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