Re: Limit + group + join

From: Jeffrey W. Baker
Subject: Re: Limit + group + join
Date: ,
Msg-id: 1125021419.16451.0.camel@noodles
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In response to: Limit + group + join  (Tobias Brox)
Responses: Re: Limit + group + join  (Tobias Brox)
Re: Limit + group + join  ("Jeffrey W. Baker")
List: pgsql-performance

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Limit + group + join  (Tobias Brox, )
 Re: Limit + group + join  ("Jeffrey W. Baker", )
  Re: Limit + group + join  (Tobias Brox, )
  Re: Limit + group + join  ("Jeffrey W. Baker", )
 Re: Limit + group + join  (Mark Kirkwood, )
  Re: Limit + group + join  (Tobias Brox, )
  Re: Limit + group + join  (Stephan Szabo, )
  Re: Limit + group + join  (Tom Lane, )
   Re: Limit + group + join  (Mark Kirkwood, )
    Re: Limit + group + join  (Tom Lane, )
   Re: Limit + group + join  (Mark Kirkwood, )
    Re: Limit + group + join  (Tom Lane, )
     Re: Limit + group + join  (Mark Kirkwood, )
      Re: Limit + group + join  (Tobias Brox, )
   Re: Limit + group + join  (Greg Stark, )
 Re: Limit + group + join  ("Merlin Moncure", )
 Re: Limit + group + join  ("Merlin Moncure", )

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



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