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help / color / mirror / Atom feedFrom: Antonin Houska <ah@cybertec.at>
To: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Fujii Masao <masao.fujii@gmail.com>
Cc: Robert Treat <rob@xzilla.net>
Cc: Pg Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Adding REPACK [concurrently]
Date: Thu, 21 Aug 2025 20:07:04 +0200
Message-ID: <40729.1755799624@localhost> (raw)
In-Reply-To: <CADzfLwW=b=U3e6aasi=XorN8hZSiCKZErKs9qhyK7m=w=wokAg@mail.gmail.com>
References: <CADzfLwXx46j8KwQjjM1ZcqNBsx-k6GxHOzDJkm4SHjh+cv31Rw@mail.gmail.com>
<202508091333.qvgvo7ikuezm@alvherre.pgsql>
<CADzfLwW=b=U3e6aasi=XorN8hZSiCKZErKs9qhyK7m=w=wokAg@mail.gmail.com>
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 = HeapTupleHeaderGetRawCommandId(tuple->t_data);
> TransactionId xid = 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 MVCC
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
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To: pgsql-hackers@postgresql.org
Cc: ah@cybertec.at, mihailnikalayeu@gmail.com, alvherre@alvh.no-ip.org, masao.fujii@gmail.com, rob@xzilla.net, pgsql-hackers@lists.postgresql.org
Subject: Re: Adding REPACK [concurrently]
In-Reply-To: <40729.1755799624@localhost>
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