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help / color / mirror / Atom feedFrom: Alvaro Herrera <alvherre@kurilemu.de>
To: Antonin Houska <ah@cybertec.at>
Cc: Rui Zhao <zhaorui126@gmail.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@lists.postgresql.org, Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Subject: Re: Race conditions in logical decoding
Date: Fri, 18 Sep 2026 14:28:55 +0200
Message-ID: <aq0sA8aF8q6kTmJM@alvherre.pgsql> (raw)
In-Reply-To: <8247.1789634255@localhost>
On 2026-Sep-17, Antonin Houska wrote:
> > + for (int i = 0; i < nrunning; i++)
> > + {
> > + TransactionId running_xid = running->xids[i];
> > +
> > + if (bsearch(&running_xid, snap->xip, snap->xcnt,
> > + sizeof(TransactionId), xidComparator) != NULL)
> > + XactLockTableWait(running_xid, NULL, NULL, XLTW_None);
> > + }
> > + }
>
> I don't understand why you check all transactions in procarray, instead of
> only those in snap->xip.
Hmm, but he does: for all the transactions that are running, only those
that are found by bsearch() in the snap->xip array are waited for. Is
that not what we want?
I guess we could do it the other way around: iterate for each item on
snap->xip and search for those in running->xids. Is that what you
suggest?
We don't know offhand which array is largest; it would be better to
iterate on the smaller one and bsearch the largest. (Or maybe if both
are sorted, scan them simultaneously.) I don't find any reference to
say that running_xid is sorted.
I don't understand these two paragraphs:
* A subtransaction is covered by its top-level transaction, which is in
* snap->xip as well, or was purged from it because it is below xmin and
* thus finished long ago.
*
* Historic snapshots do not need this: between xmin and xmax they rely on
* xip alone, and transactions below xmin had left the procarray by the
* time the xl_running_xacts record that set xmin was written.
--
Álvaro Herrera PostgreSQL Developer — https://www.EnterpriseDB.com/
"After a quick R of TFM, all I can say is HOLY CR** THAT IS COOL! PostgreSQL was
amazing when I first started using it at 7.2, and I'm continually astounded by
learning new features and techniques made available by the continuing work of
the development team."
Berend Tober, http://archives.postgresql.org/pgsql-hackers/2007-08/msg01009.php
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Cc: alvherre@kurilemu.de, ah@cybertec.at, zhaorui126@gmail.com, andres@anarazel.de, mihailnikalayeu@gmail.com
Subject: Re: Race conditions in logical decoding
In-Reply-To: <aq0sA8aF8q6kTmJM@alvherre.pgsql>
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