problem with the Index

I have a problem with the index of 1 table.

I hava a table created :
    CREATE TABLE "acucliart" (
   "cod_pto" numeric(8,0) NOT NULL,
   "cod_cli" varchar(9) NOT NULL,
   "mes" numeric(2,0) NOT NULL,
   "ano" numeric(4,0) NOT NULL,
   "int_art" numeric(5,0) NOT NULL,
   "cantidad" numeric(12,2),
   "ven_siv_to" numeric(14,2),
   "ven_civ_to" numeric(14,2),
   "tic_siv_to" numeric(14,2),
   "tic_civ_to" numeric(14,2),
   "visitas" numeric(2,0),
   "ult_vis" date,
   "ven_cos" numeric(12,2),
   "ven_ofe" numeric(12,2),
   "cos_ofe" numeric(12,2),
   CONSTRAINT "acucliart_pkey"
   PRIMARY KEY ("cod_cli")
);

if i do this select:
    explain select * from acucliart where cod_cli=10000;
        postgres use the index
        NOTICE:  QUERY PLAN:
        Index Scan using cod_cli_ukey on acucliart  (cost=0.00..4.82 rows=1
width=478)

and this select
        explain select * from acucliart where cod_cli>10000;
        Postgres don't use the index:
        NOTICE:  QUERY PLAN:
        Seq Scan on acucliart  (cost=0.00..22.50 rows=333 width=478)

why?


tk