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From: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Date: Fri, 28 Nov 2025 15:50:00 +0100
Message-ID: 
 <CADzfLwUKXcXKZgX+e8ACsOXe_CgtWmNJY_6dyn8EO0AXYOn2pA@mail.gmail.com>
Subject: Re: Revisiting {CREATE INDEX, REINDEX} CONCURRENTLY improvements
To: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Sergey Sargsyan <sergey.sargsyan.2001@gmail.com>,
	=?UTF-8?Q?=C3=81lvaro_Herrera?= <alvherre@kurilemu.de>,
	Andres Freund <andres@anarazel.de>, Michael Paquier <michael@paquier.xyz>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
 Andrey Borodin <amborodin86@gmail.com>,
	Melanie Plageman <melanieplageman@gmail.com>
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Hello!

On Thu, Nov 27, 2025 at 9:07=E2=80=AFPM Matthias van de Meent
<boekewurm+postgres@gmail.com> wrote:
> While it might not break, and might not hold back other tables'
> visibility horizons, it'll still hold back pruning on the table we're
> acting on, and that's likely one which already had bloat issues if
> you're running RIC (or REPACK).

Yes, a good point about REPACK, agreed.

BTW, what is about using the same reset snapshot technique for REPACK also?

I thought it is impossible, but what if we:

* while reading the heap we "remember" our current page position into
shared memory
* preserve all xmin/max/cid into newly created repacked table (we need
it for MVCC-safe approach anyway)
* in logical decoding layer - we check TID of our tuple and looking at
"current page" we may correctly decide what to do with at apply phase:

- if it in "non-yet read pages" - ignore (we will read it later) - but
signal scan to ensure it will reset snapshot before that page
(reset_before =3D min(reset_before, tid))
- if it in "already read pages" - remember the apply operation (with
exact target xmin/xmax and resulting xmin/xmax)

 Before switching table - use the same "limit_xmin" logic to wait for
other transactions the same way CIC does.

It may involve some tricky locking, maybe I missed some cases, but it
feels like it is possible to do it correctly by combining information
of scan state and xmin/xmax/tid/etc...

PS.

> PS. When I checked the code you linked to on that thread, I noticed
> there is a stale pointer dereference issue in
> GetPinnedOldestNonRemovableTransactionId, where it pulls data from a
> hash table entry that could've been released by that point.

Thanks!