Subject: Re: Limit + group + join
From: "Jeffrey W. Baker" <jwbaker@acm.org>
To: Tobias Brox <tobias@nordicbet.com>
Cc: pgsql-performance@postgresql.org
In-Reply-To: <1125021419.16451.0.camel@noodles>
References: <20050826002709.GK10328@tobias.lan>
	<1125021419.16451.0.camel@noodles>
Content-Type: text/plain
Date: Thu, 25 Aug 2005 19:31:20 -0700
Message-Id: <1125023480.16451.2.camel@noodles>
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On Thu, 2005-08-25 at 18:56 -0700, Jeffrey W. Baker wrote:
> 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.

Nevermind.  I read c_id as c.id.

-jwb