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: <20050826002709.GK10328@tobias.lan>
References: <20050826002709.GK10328@tobias.lan>
Content-Type: text/plain
Date: Thu, 25 Aug 2005 18:56:59 -0700
Message-Id: <1125021419.16451.0.camel@noodles>
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit

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