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

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От Merlin Moncure
Тема is it possible to for the planner to optimize this form?
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Msg-id 6EE64EF3AB31D5448D0007DD34EEB34101AE56@Herge.rcsinc.local
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Ответы Re: is it possible to for the planner to optimize this form?  (Josh Berkus)
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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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