cpu_tuple_cost

Hi List,

i have a query plan who is bad with standard cpu_tuple_costs and good if
I raise cpu_tuple_costs. Is it is a good practice to raise them if i
want to force postgres to use indexes more often? Or is it is better to
disable sequence scans?

CIMSOFT=# ANALYSE mitpln;
ANALYZE

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

--------------------------------------------------------------------------------
Seq Scan on mitpln (cost=0.00..1411.85 rows=2050 width=69) (actual
time=562.000..1203.000 rows=1269 loops=1)
Filter: ((date_to_yearmonth_dec((mpl_date)::timestamp without time
zone))::text = '20050'::text)
Total runtime: 1203.000 ms
(3 rows)

CIMSOFT=# SET cpu_tuple_cost = 0.07;
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..2962.86
rows=2050width=69) (actual time=0.000..0.000 rows=1269 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))

CIMSOFT=# SELECT count(*) FROM mitpln;
count
-------
26128
(1 row)