is it possible to for the planner to optimize this form?

От: Merlin Moncure
Тема: is it possible to for the planner to optimize this form?
Дата: ,
Msg-id: 6EE64EF3AB31D5448D0007DD34EEB34101AE56@Herge.rcsinc.local
(см: обсуждение, исходный текст)
Ответы: Re: is it possible to for the planner to optimize this form?  (Josh Berkus)
Список: pgsql-performance

Right now, I am having trouble getting the planner to optimize queries
in the form of

select t.key, t.field from t a
    where
    (
        select count(*) from t b
        where b.field > a.field
    ) = k

The subplan (either index or seq. scan) executes once for each row in t,
which of course takes forever.

This query is a way of achieving LIMIT type results (substitute n-1
desired rows for k) using standard SQL, which is desirable in some
circumstances.  Is it theoretically possible for this to be optimized?

Merlin



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