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 "Mon, 25 Aug 2025 18:23:09 +0200."
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Date: Mon, 25 Aug 2025 19:22:14 +0200
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Mihail Nikalayeu <mihailnikalayeu@gmail.com> wrote:

> Hi, Antonin
> =

> > How does HeapTupleSatisfiesSelf() recognize the status of any XID w/o =
using a
> > snapshot? Do you mean by checking the commit log (TransactionIdDidComm=
it) ?
> =

> Yes, TransactionIdDidCommit.

I think the problem is that HeapTupleSatisfiesSelf() uses
TransactionIdIsInProgress() instead of checking the snapshot:

        ...
        else if (TransactionIdIsInProgress(HeapTupleHeaderGetRawXmin(tuple=
)))
                return false;
        else if (TransactionIdDidCommit(HeapTupleHeaderGetRawXmin(tuple)))
        ...

When decoding (and replaying) data changes, you deal with the database sta=
te
as it was (far) in the past. However TransactionIdIsInProgress() is not
suitable for this purpose.

And since CommitTransaction() updates the commit log before removing the
transaction from ProcArray, I can even imagine race conditions: if a
transaction is committed and decoded fast enough, TransactionIdIsInProgres=
s()
might still return true. In such a case, HeapTupleSatisfiesSelf() returns
false instead of calling TransactionIdDidCommit().

> Another option is just invent a new
> snapshot type - SnapshotBelieveEverythingCommitted - for that
> particular case it should work - because all xmin/xmax written into
> the new table are committed by design.

I'd prefer optimization of the logical decoding for REPACK CONCURRENTLY, a=
nd
using the MVCC snapshots.

-- =

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