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In-Reply-To: <71E42FAF-FF92-4EE5-94A5-341EFB891281@yandex-team.ru>
From: Alexander Korotkov <aekorotkov@gmail.com>
Date: Sun, 3 May 2026 22:44:32 +0300
Message-ID: <CAPpHfdsoVkyAwTr4E6pt0O42Cchxy3JuCzBWesxv03KmxAyP4A@mail.gmail.com>
Subject: Re: [BUG] false positive in bt_index_check in case of short 4B
 varlena datum
To: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: Andres Freund <andres@anarazel.de>, Alexander Lakhin <exclusion@gmail.com>, 
	Michael Zhilin <m.zhilin@postgrespro.ru>, pgsql-bugs@postgresql.org, 
	Yura Sokolov <y.sokolov@postgrespro.ru>
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On Sun, May 3, 2026 at 9:17=E2=80=AFPM Andrey Borodin <x4mmm@yandex-team.ru=
> wrote:
> > On 2 May 2026, at 00:41, Andres Freund <andres@anarazel.de> wrote:
> >
> > This is checking (as you noted) !VARATT_IS_EXTENDED, whereas the
> > bt_normalize_tuple() code is checking !VARATT_IS_COMPRESSED.
> >
> > VARATT_IS_EXTENDED() will return true for short varlenas (because it's =
not a
> > standard 4 byte uncompressed varlena), whereas VARATT_IS_COMPRESSED() w=
ill
> > return false for a short varlena (since it's not compressed).
> >
> > I didn't find other instanes of similar code that uses !VARATT_IS_COMPR=
ESSED.
>
> As far as I understand
>
> !VARATT_IS_COMPRESSED(DatumGetPointer(normalized[i])) && !VARATT_IS_SHORT=
(DatumGetPointer(normalized[i]))
>
> is exactly
>
> !VARATT_IS_EXTENDED(DatumGetPointer(untoasted_values[i]))
>
> Which is what was proposed in v2 patch. But later was changed to !VARATT_=
IS_COMPRESSED().
> As I understood it was done to further strengthen normalization.
>
> So the intent might be that short varatts need normalization in some case=
s. But we have no tests that show such a case.
> I tried to build a problematic storage alternation like [0], but everythi=
ng works nicely.
>
> So I propose something in a line with attached patch.
>
>
> Best regards, Andrey Borodin.
>
> [0] https://github.com/postgres/postgres/blob/master/contrib/amcheck/sql/=
check_btree.sql#L170-L172

AFAICS, this is correct.  However, I propose an alternative approach:
use VARSIZE_ANY().  This might be a bit slower, but looks more
intuitive to me.

------
Regards,
Alexander Korotkov
Supabase

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