Why isn't an index being used when selecting a distinct value?

Version: Postgres 8.1.4
Platform: RHEL

Given this scenario with the indexes in place, when I ask for the distinct
field1_id values, why does the optimizer choose a sequential scan instead of
just reading from the kda_log_fid_cre_20080123_idx index? The time it takes
to perform the sequential scan against 20+ million records is way too slow.

CREATE TABLE kda_log_20080213 (
"field1" character varying(255),
field character varying(100),
value bigint,
period integer DEFAULT 60,
created timestamp with time zone DEFAULT ('now'::text)::timestamp(6)
with time zone,
customer_id integer,
field1_id integer
);

CREATE INDEX kda_log_cid_cre_fld_20080213_idx ON kda_log_20080213 USING
btree (customer_id, created, "field1");

CREATE INDEX kda_log_fid_cre_20080213_idx ON kda_log_20080213 USING btree
(field1_id, created);

keaton=# explain select distinct field1_id into temp kda_temp from
kda_log_20080213;
QUERY PLAN
----------------------------------------------------------------------------
------------------
Unique (cost=5759201.93..5927827.87 rows=8545 width=4)
-> Sort (cost=5759201.93..5843514.90 rows=33725188 width=4)
Sort Key: field1_id
-> Seq Scan on kda_log_20080213 (cost=0.00..748067.88
rows=33725188 width=4)
(4 rows)

Thanks,

Keaton

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