Feedback-ID: id4a34324:Fastmail
Date: Fri, 1 May 2026 15:41:05 -0400
From: Andres Freund <andres@anarazel.de>
To: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: Alexander Korotkov <aekorotkov@gmail.com>, 
	Alexander Lakhin <exclusion@gmail.com>, Michael Zhilin <m.zhilin@postgrespro.ru>, 
	pgsql-bugs@postgresql.org, Yura Sokolov <y.sokolov@postgrespro.ru>
Subject: Re: [BUG] false positive in bt_index_check in case of short 4B
 varlena datum
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Hi,

On 2026-05-01 23:06:49 +0500, Andrey Borodin wrote:
> > On 1 May 2026, at 22:11, Andres Freund <andres@anarazel.de> wrote:
> > 
> > While hacking on something, I added an assertion to VARSIZE() that the
> > argument is actually a VARATT_4B (which it assumes). Worked everywhere, except
> > for this caller: amcheck/regress fails, because sometimes the varlena is
> > actually a short/1B varlena.
> > 
> > Note that VARSIZE_4B on a short datum will give you completely bogus
> > answers. E.g. in the case that failed the assertion, VARSIZE_1B() is 2, but
> > VARSIZE_4B(PTR) is 7681.
> 
> I remember the original code was taken from somewhere else because there
> was already some instances like this:

> /*
>  * If value is above size target, and is of a compressible datatype,
>  * try to compress it in-line.
>  */
> if (!VARATT_IS_EXTENDED(DatumGetPointer(untoasted_values[i])) &&
> VARSIZE(DatumGetPointer(untoasted_values[i])) > TOAST_INDEX_TARGET &&
> (att->attstorage == TYPSTORAGE_EXTENDED ||
> att->attstorage == TYPSTORAGE_MAIN))
> {
> 
> I don't have VARATT_IS_EXTENDED vs VARATT_IS_COMPRESSED vs VARATT_IS_SHORT
> business in my warm cache right away, but I'll try to remember what it means soon.

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() will
return false for a short varlena (since it's not compressed).

I didn't find other instanes of similar code that uses !VARATT_IS_COMPRESSED.

Greetings,

Andres Freund