From: Antonin Houska <ah@cybertec.at>
To: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
    Fujii Masao <masao.fujii@gmail.com>, Robert Treat <rob@xzilla.net>,
    Pg Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Adding REPACK [concurrently]
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Comments: In-reply-to Mihail Nikalayeu <mihailnikalayeu@gmail.com>
   message dated "Thu, 21 Aug 2025 01:44:00 +0200."
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Date: Thu, 21 Aug 2025 20:07:04 +0200
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Mihail Nikalayeu <mihailnikalayeu@gmail.com> wrote:

> Also, I think I found an issue (or lost something during rebase): we
> must preserve xmin,cmin during initial copy
> to make sure that data is going to be visible by snapshots of
> concurrent changes later:
> =

> static void
> reform_and_rewrite_tuple(......)
> .....
>       /*It is also crucial to stamp the new record with the exact same
> xid and cid,
>       * because the tuple must be visible to the snapshot of the
> applied concurrent
>       * change later.
>       */
>       CommandId      cid =3D HeapTupleHeaderGetRawCommandId(tuple->t_dat=
a);
>       TransactionId   xid =3D HeapTupleHeaderGetXmin(tuple->t_data);
> =

>       heap_insert(NewHeap, copiedTuple, xid, cid, HEAP_INSERT_NO_LOGICAL=
, NULL);

When posting version 12 of the patch [1] I raised a concern that the the M=
VCC
safety is too expensive when it comes to logical decoding. Therefore, I
abandoned the concept for now, and v13 [2] uses plain heap_insert(). Once =
we
implement the MVCC safety, we simply rewrite the tuple like v12 did - that=
's
the simplest way to preserve fields like xmin, cmin, ...

[1] https://www.postgresql.org/message-id/178741.1743514291%40localhost
[2] https://www.postgresql.org/message-id/97795.1744363522%40localhost

-- =

Antonin Houska
Web: https://www.cybertec-postgresql.com