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)
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