Re: Limit + group + join

Поиск
Список
Период
Сортировка
От Jeffrey W. Baker
Тема Re: Limit + group + join
Дата
Msg-id 1125021419.16451.0.camel@noodles
обсуждение исходный текст
Ответ на Limit + group + join  (Tobias Brox <tobias@nordicbet.com>)
Ответы Re: Limit + group + join  (Tobias Brox <tobias@nordicbet.com>)
Re: Limit + group + join  ("Jeffrey W. Baker" <jwbaker@acm.org>)
Список pgsql-performance
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


В списке pgsql-performance по дате отправления:

Предыдущее
От: Tobias Brox
Дата:
Сообщение: Limit + group + join
Следующее
От: Tobias Brox
Дата:
Сообщение: Re: Limit + group + join