Re: deferred writing of two-phase state files adds fragility

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From: Robert Haas <[email protected]>
To: Tom Lane <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: deferred writing of two-phase state files adds fragility
Date: Wed, 4 Dec 2024 13:01:42 -0500
Message-ID: <CA+Tgmoa7ytdSS1U2ZXNHr+mNgVwhnwOWp2g3pkm1Vu-RvA8M+w@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <CA+Tgmob2e542abFO-RspquqVYzpt7X4JeOKMDVXwDEowqzmcOg@mail.gmail.com>
	<[email protected]>
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On Wed, Dec 4, 2024 at 12:58 PM Tom Lane <[email protected]> wrote:
> Robert Haas <[email protected]> writes:
> > On Wed, Dec 4, 2024 at 12:19 PM Tom Lane <[email protected]> wrote:
> >> Could we fix it so ROLLBACK TRANSACTION removes the GID from the
> >> list of prepared xacts that need to be written out?  Then we'd
> >> no longer have a pending requirement to read the broken WAL record.
>
> > That would be nice, but I'm not sure that it's possible. As currently
> > implemented, FinishPreparedTransaction() always reads the two-phase
> > state data either from the two-phase file or the WAL, whether it's
> > committing or rolling back.
>
> I'm not following.  FinishPreparedTransaction is not what's preventing
> checkpoints or holding back the VACUUM horizon.  What is doing that
> is the in-memory fake PGPROC representing the prepared transaction
> (I forget the exact terminology).  I'm suggesting that we could have
> some way to nuke one of those without properly cleaning up the
> prepared xact.  Maybe it'd need to be invoked via a different command
> than ROLLBACK TRANSACTION.

Yes, that we could do. Perhaps it could be added to pg_surgery.

-- 
Robert Haas
EDB: http://www.enterprisedb.com

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