From: Tom Lane <tgl@sss.pgh.pa.us>
To: =?UTF-8?Q?Andr=C3=A9_H=C3=A4nsel?= <andre@webkr.de>
cc: "'Pgsql Performance'" <pgsql-performance@lists.postgresql.org>
Subject: Re: Why is there a Sort after an Index Only Scan?
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Comments: In-reply-to =?UTF-8?Q?Andr=C3=A9_H=C3=A4nsel?= <andre@webkr.de>
	message dated "Thu, 05 May 2022 03:12:40 +0200"
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Date: Wed, 04 May 2022 22:08:59 -0400
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=3D?UTF-8?Q?Andr=3DC3=3DA9_H=3DC3=3DA4nsel?=3D <andre@webkr.de> writes:
> Limit  (cost=3D0.43..296.63 rows=3D50 width=3D4) (actual time=3D1052.692=
..1052.737 rows=3D50 loops=3D1)
>   ->  Index Scan using orders_test_pkey on orders_test  (cost=3D0.43..71=
149.43 rows=3D12010 width=3D4) (actual time=3D1052.690..1052.728 rows=3D50=
 loops=3D1)
>         Filter: ((shipping_date >=3D '2022-04-30'::date) AND (shipping_d=
ate <=3D '2022-05-01'::date))
>         Rows Removed by Filter: 1998734

> Is it overestimating the cost of the sorting?

No, but it's guessing it will hit 50 rows that satisfy the filter
before it's gone very far in this index.  If the shipping date and
pkey are correlated in the wrong direction, that could be a very
overoptimistic guess.  I don't think we have adequate stats yet
to detect this sort of problem.

			regards, tom lane