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From: "Gaetano Mendola" <mendola@bigfoot.com>
X-Newsgroups: comp.databases.postgresql.sql
Subject: Performance inside and outside view ( WAS Re: Select the max on a
	field )
Date: Thu, 12 Sep 2002 16:31:04 +0200
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"Stephan Szabo" <sszabo@megazone23.bigpanda.com> wrote in message
news:20020912065516.L45340-100000@megazone23.bigpanda.com...
> select distinct on (att_2) * from test
> order by att_2, att_1 desc;

Yes that's right it's help me,
but here the optimizer have some problems:


CREATE VIEW last_user_logs AS
SELECT DISTINCT ON (id_user) *
FROM user_logs
ORDER BY id_user, id_user_log DESC;

and now:

# explain analyze select * from user_logs where id_user = '5430';
NOTICE:  QUERY PLAN:

Index Scan using idx_user_user_logs on user_logs  (cost=0.00..3038.59
rows=939 width=68) (actual time=0.03..0.03 rows=0 loops=1)
Total runtime: 0.12 msec

EXPLAIN

empdb=# explain analyze select * from last_user_logs where id_user = 5430;
NOTICE:  QUERY PLAN:

Subquery Scan last_user_logs  (cost=20256.12..20725.43 rows=18772 width=68)
(actual time=3526.10..3526.10 rows=0 loops=1)
  ->  Unique  (cost=20256.12..20725.43 rows=18772 width=68) (actual
time=3067.14..3522.54 rows=2226 loops=1)
        ->  Sort  (cost=20256.12..20256.12 rows=187723 width=68) (actual
time=3067.13..3241.94 rows=187723 loops=1)
              ->  Seq Scan on user_logs  (cost=0.00..3813.23 rows=187723
width=68) (actual time=0.02..1070.59 rows=187723 loops=1)
Total runtime: 3578.07 msec

EXPLAIN


if I do instead without view and
#explain analyze SELECT DISTINCT ON (id_user) *
  FROM user_logs
 WHERE id_user = 5430
 ORDER BY id_user, id_user_log DESC;

NOTICE:  QUERY PLAN:

Unique  (cost=3084.93..3087.27 rows=94 width=68) (actual time=0.21..0.21
rows=0 loops=1)
  ->  Sort  (cost=3084.93..3084.93 rows=939 width=68) (actual
time=0.21..0.21 rows=0 loops=1)
        ->  Index Scan using idx_user_user_logs on user_logs
(cost=0.00..3038.59 rows=939 width=68) (actual time=0.03..0.03 rows=0
loops=1)
Total runtime: 0.31 msec

EXPLAIN


Ciao
Gaetano