Re: cpu_tuple_cost - Mailing list pgsql-performance

Tom Lane wrote:

>
> Reducing random_page_cost is usually the best way to get the planner to
> favor indexscans more.
>

Ok, I tried a bit with random_page_cost and I have set it to 1 to become
  PG using the index on mitpln:

CIMSOFT=# ANALYSE mitpln;
ANALYZE
CIMSOFT=# SET random_page_cost=2;
SET
CIMSOFT=# EXPLAIN ANALYSE SELECT * FROM mitpln WHERE
date_to_yearmonth_dec(mpl_date)='20050';
                                                   QUERY PLAN

--------------------------------------------------------------------------------

  Seq Scan on mitpln  (cost=0.00..1173.78 rows=1431 width=69) (actual
time=219.000..1125.000 rows=1266 loops=1)
    Filter: ((date_to_yearmonth_dec((mpl_date)::timestamp without time
zone))::text = '20050'::text)
  Total runtime: 1125.000 ms
(3 rows)

CIMSOFT=# SET random_page_cost=1;
SET
CIMSOFT=# EXPLAIN ANALYSE SELECT * FROM mitpln WHERE
date_to_yearmonth_dec(mpl_date)='20050';
                                                               QUERY PLAN

--------------------------------------------------------------------------------
  Index Scan using mitpln_yearmonth_dec on mitpln  (cost=0.00..699.01
rows=1431 width=69) (actual time=0.000..16.000 rows=1266 loops=1)
    Index Cond: ((date_to_yearmonth_dec((mpl_date)::timestamp without
time zone))::text = '20050'::text)
  Total runtime: 16.000 ms
(3 rows)


CIMSOFT=# \d mitpln
                                        Table "public.mitpln"
     Column    |         Type          |                         Modifiers

--------------+-----------------------+-----------------------------------------
  mpl_id       | integer               | not null default
nextval('public.mitpln_mpl_id_seq'::text)
  mpl_date     | date                  |
  mpl_minr     | integer               | not null
  mpl_tpl_name | character varying(20) |
  mpl_feiertag | character varying(50) |
  mpl_min      | real                  |
  mpl_saldo    | real                  |
  mpl_buch     | boolean               | not null default false
  mpl_absaldo  | real                  |
  mpl_vhz      | real                  |
  dbrid        | character varying     | default nextval('db_id_seq'::text)
Indexes:
     "mitpln_pkey" PRIMARY KEY, btree (mpl_id)
     "mitpln_idindex" UNIQUE, btree (dbrid)
     "xtt5126" UNIQUE, btree (mpl_date, mpl_minr)
     "mitpln_yearmonth_dec" btree
(date_to_yearmonth_dec(mpl_date::timestamp with
out time zone))
     "mpl_minr" btree (mpl_minr)
     "mpl_minr_nobuch" btree (mpl_minr) WHERE NOT mpl_buch


CIMSOFT=# SELECT count(*) FROM mitpln;
  count
-------
  26330
(1 row)


CREATE OR REPLACE FUNCTION date_to_yearmonth_dec(TIMESTAMP) RETURNS
VARCHAR AS'
BEGIN
  RETURN extract(year FROM $1) || extract(month FROM $1)-1;
END'LANGUAGE plpgsql IMMUTABLE;


Daniel

PS : thats a 2.4 GHZ P4 Server with 1 GB Ram and RAID - SCSI
(WIN2000, PG8.0.1)