From: | David Rowley <dgrowleyml(at)gmail(dot)com> |
---|---|
To: | Dimitrios Apostolou <jimis(at)gmx(dot)net> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-general(at)lists(dot)postgresql(dot)org |
Subject: | Re: SELECT DISTINCT chooses parallel seqscan instead of indexscan on huge table with 1000 partitions |
Date: | 2024-05-13 13:32:02 |
Message-ID: | CAApHDvo8yYvqa1+bkW_f5xHX-gmKGYfaGwH+Y_KP-=9TOuF+-g@mail.gmail.com |
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On Tue, 14 May 2024 at 00:41, Dimitrios Apostolou <jimis(at)gmx(dot)net> wrote:
>
> On Sat, 11 May 2024, David Rowley wrote:
> > It will. It's just that Sorting requires fetching everything from its subnode.
>
> Isn't it plain wrong to have a sort step in the plan than? The different
> partitions contain different value ranges with no overlap, and the last
> query I posted doesn't even contain an ORDER BY clause, just a DISTINCT
> clause on an indexed column.
The query does contain an ORDER BY, so if the index is not chosen to
provide pre-sorted input, then something has to put the results in the
correct order before the LIMIT is applied.
> Even with bad estimates, even with seq scan instead of index scan, the
> plan should be such that it concludes all parallel work as soon as it
> finds the 10 distinct values. And this is actually achieved if I disable
> parallel plans. Could it be a bug in the parallel plan generation?
If you were to put the n_distinct_inherited estimate back to 200 and
disable sort, you should see the costs are higher for the index plan.
If that's not the case then there might be a bug. It seems more
likely that due to the n_distinct estimate being so low that the
planner thought that a large enough fraction of the rows needed to be
read and that made the non-index plan appear cheaper.
I'd be interested in seeing what the costs are for the index plan. I
think the following will give you that (untested):
alter table test_runs_raw alter column workitem_n set
(n_distinct_inherited=200);
analyze test_runs_raw;
set enable_sort=0;
explain SELECT DISTINCT workitem_n FROM test_runs_raw ORDER BY
workitem_n DESC LIMIT 10;
-- undo
alter table test_runs_raw alter column workitem_n set (n_distinct_inherited=-1);
reset enable_sort;
David
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