From: Jeffrey W. Baker <jwbaker@acm.org>
To: Tobias Brox <tobias@nordicbet.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Limit + group + join
Date: Thu, 25 Aug 2005 18:56:59 -0700
Message-ID: <1125021419.16451.0.camel@noodles> (raw)
In-Reply-To: <20050826002709.GK10328@tobias.lan>
References: <20050826002709.GK10328@tobias.lan>
On Fri, 2005-08-26 at 02:27 +0200, Tobias Brox wrote:
> Consider this setup - which is a gross simplification of parts of our
> production system ;-)
>
> create table c (id integer primary key);
> create table b (id integer primary key, c_id integer);
> create index b_on_c on b(c_id)
>
> insert into c (select ... lots of IDs ...);
> insert into b (select id, id from c); /* keep it simple :-) */
>
> Now, I'm just interessted in some few rows.
>
> All those gives good plans:
>
> explain select c.id from c order by c.id limit 1;
> explain select c.id from c group by c.id order by c.id limit 1;
> explain select c.id from c join b on c_id=c.id order by c.id limit 1;
>
> ... BUT ... combining join, group and limit makes havoc:
>
> explain select c.id from c join b on c_id=c.id group by c.id order by c.id
> desc limit 5;
Where's b in this join clause? It looks like a cartesian product to me.
-jwb
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: jwbaker@acm.org, tobias@nordicbet.com
Subject: Re: Limit + group + join
In-Reply-To: <1125021419.16451.0.camel@noodles>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox