Thread: Re: is it possible to for the planner to optimize this

I didn't really look that closely at the problem but have you thought of trying:

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

This is called an inline view or sometimes a nested table.  You would be joining table t to this inline view with the join criteria being "cntb = k" where k is in t.

-----Original Message-----
From: Josh Berkus [mailto:josh@agliodbs.com]
Sent: Monday, June 07, 2004 1:32 PM
To: Merlin Moncure; pgsql-performance@postgresql.org
Subject: Re: [PERFORM] is it possible to for the planner to optimize
this form?

Merlin,

> 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?

I don't think so, no.   PostgreSQL does have some issues using indexes for
count() queires which makes the situation worse.   However, with the query
you presented, I don't see any way around the planner executing the subquery
once for every row in t.

Except, of course, for some kind of scheme involving materialized views, if
you don't need up-to-the minute data.   In that case, you could store in a
table the count(*)s of t for each threshold value of b.field.  But,
dynamically, that would be even slower.

--
Josh Berkus
Aglio Database Solutions
San Francisco

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