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

[Keaton Adams <kadams@xxxxxxxxxxx>]

Title: 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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