Hello,
I just installed PG 7.0 and did some testing. I found what I believe is
a strange behaviour:
xtnet=# \d telecom Table "telecom"Attribute | Type | Modifier
-----------+-------------+----------regione | varchar(20) | distretto | varchar(20) | settore | varchar(20) |
descriz | varchar(60) | cor_x | float8 | cor_y | float8 | cod_dis | varchar(4) | rete_urb |
varchar(24)|
xtnet=# create index telecom_regione on telecom ( regione );
CREATE
xtnet=# explain select distinct on (regione) * from telecom;
NOTICE: QUERY PLAN:
Unique (cost=0.00..4777.43 rows=3910 width=88) -> Index Scan using telecom_regione on telecom (cost=0.00..4679.68
rows=39100 width=88)
EXPLAIN
xtnet=# explain select distinct on (regione) regione from telecom;
NOTICE: QUERY PLAN:
Unique (cost=4370.91..4468.66 rows=3910 width=12) -> Sort (cost=4370.91..4370.91 rows=39100 width=12) -> Seq
Scanon telecom (cost=0.00..1079.00 rows=39100
width=12)
EXPLAIN
xtnet=# explain select distinct on (regione) distretto,regione from
telecom;
NOTICE: QUERY PLAN:
Unique (cost=4492.72..4590.47 rows=3910 width=24) -> Sort (cost=4492.72..4492.72 rows=39100 width=24) -> Seq
Scanon telecom (cost=0.00..1079.00 rows=39100
width=24)
EXPLAIN
It seems like the index is used only if * appears in the target list.
???
Bye!
-- Daniele
-------------------------------------------------------------------------------Daniele Orlandi - Utility Line Italia -
http://www.orlandi.comViaMezzera 29/A - 20030 - Seveso (MI) - Italy
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