Re: Limit + group + join

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