On Fri, Aug 8, 2014 at 4:05 AM, Evgeniy Shishkin <itparan...@gmail.com> wrote: >>>>> select * from users join notifications on users.id=notifications.user_id >>>>> ORDER BY users.priority desc ,notifications.priority desc limit 10;
>>>> In my understanding, i need to have two indexes >>>> on users(priority desc, id) >>>> and notifications(user_id, priority desc) > And actually with this kind of query we really want the most wanted > notifications, by the user. > So we really can rewrite to order by users.priority desc, id asc, > notifications.priority desc according to business logic. You can rewrite it with LATERAL to trick the planner into sorting each user's notifications separately. This should give you the nestloop plan you expect: SELECT * FROM users, LATERAL ( SELECT * FROM notifications WHERE notifications.user_id=users.id ORDER BY notifications.priority DESC ) AS notifications ORDER BY users.priority DESC, users.id It would be great if Postgres could do this transformation automatically. There's a "partial sort" patch in the current CommitFest, which would solve the problem partially (it could use the index on users, but the notifications sort would have to be done in memory still). https://commitfest.postgresql.org/action/patch_view?id=1368 Regards, Marti -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
