Message-ID: <5169168D.4000903@ngs.hr>
Date: Sat, 13 Apr 2013 10:25:49 +0200
From: Rikard Pavelic <rikard@ngs.hr>
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64;
	rv:8.0) Gecko/20111105 Thunderbird/8.0
MIME-Version: 1.0
To: pgsql-performance@postgresql.org
Subject: limit is sometimes not pushed in view with order
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

I was investigating some performance issues and stumbled upon this behavior:

create table main_table (i serial primary key, data varchar, ord int);
create view main_view_order as select m.i, m.data, m.ord from main_table m order by m.i desc;

insert into main_table select i, i::text, i/10 from generate_series(1,1000000) i;

create index ix_ord on main_table(ord);
analyze main_table;

explain analyze select * from main_view_order m where m.ord >= 5000 and m.ord <= 5500 limit 10;

Limit  (cost=0.00..69.01 rows=10 width=14) (actual time=330.943..330.951 rows=10 loops=1)
  ->  Index Scan Backward using main_table_pkey on main_table m  (cost=0.00..36389.36 rows=5281 width=14) (actual time=330.937..330.940 rows=10 loops=1)
        Filter: ((ord >= 5000) AND (ord <= 5500))
Total runtime: 330.975 ms

I havent found it on TODO or in archives so I'm wondering if this is a known behavior.

Regards,
Rikard

-- 
Rikard Pavelic
http://www.ngs.hr/


-- 
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance