How to use index in WHERE int = float

I have table with index

CREATE TABLE firma2.dok(
...
  dokumnr serial NOT NULL,
...
  CONSTRAINT dok_pkey PRIMARY KEY (dokumnr),
...
);

CREATE INDEX dok_dokumnr_idx
  ON firma2.dok
  USING btree
  (dokumnr);

I ran analyze command on it.

explain analyze select * from firma2.dok where  dokumnr='1228137'::float8

"Seq Scan on dok  (cost=0.00..187766.23 rows=6255 width=1145) (actual
time=43168.460..43176.063 rows=1 loops=1)"
"  Filter: ((dokumnr)::double precision = 1228137::double precision)"
"Total runtime: 43176.375 ms"

shows that index is not used and thus query takes very long time.

How to force PostgreSql to speed up without changing query ?

"PostgreSQL 8.1.4 on i686-pc-linux-gnu, compiled by GCC
i686-pc-linux-gnu-gcc (GCC) 3.4.6 (Gentoo 3.4.6-r1, ssp-3.4.5-1.0,
pie-8.7.9)"

Andrus.