From: Antonin Houska <ah@cybertec.at>
To: Amit Kapila <amit.kapila16@gmail.com>
cc: Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
    Andres Freund <andres@anarazel.de>,
    Alvaro Herrera <alvherre@alvh.no-ip.org>,
    Srinath Reddy Sadipiralla <srinath2133@gmail.com>,
    Matthias van de Meent <boekewurm+postgres@gmail.com>,
    Pg Hackers <pgsql-hackers@lists.postgresql.org>,
    Robert Treat <rob@xzilla.net>
Subject: Re: Adding REPACK [concurrently]
In-reply-to: 
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References: 
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Comments: In-reply-to Amit Kapila <amit.kapila16@gmail.com>
   message dated "Tue, 12 May 2026 13:27:29 +0530."
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Date: Tue, 12 May 2026 13:08:20 +0200
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Amit Kapila <amit.kapila16@gmail.com> wrote:

> On Tue, May 12, 2026 at 1:00=E2=80=AFAM Antonin Houska <ah@cybertec.at> w=
rote:
> >
> > Antonin Houska <ah@cybertec.at> wrote:
> >
> > > Amit Kapila <amit.kapila16@gmail.com> wrote:
> > >
> > > > On Tue, May 5, 2026 at 6:17=E2=80=AFPM Antonin Houska <ah@cybertec.=
at> wrote:
> > > > >
> > > > > Antonin Houska <ah@cybertec.at> wrote:
> > > > >
> > > > > I think the problem is that with database-specific snapshot,
> > > > > SnapBuildProcessRunningXacts() returns early, w/o adjusting build=
er->xmin
> > > > >
> > > > >         /*
> > > > >          * Database specific transaction info may exist to reach =
CONSISTENT state
> > > > >          * faster, however the code below makes no use of it. Mor=
eover, such
> > > > >          * record might cause problems because the following norm=
al (cluster-wide)
> > > > >          * record can have lower value of oldestRunningXid. In th=
at case, let's
> > > > >          * wait with the cleanup for the next regular cluster-wid=
e record.
> > > > >          */
> > > > >         if (OidIsValid(running->dbid))
> > > > >                 return;
> > > > >
> > > > > and thus some transactions whose XID is below running->oldestRunn=
ingXid may
> > > > > continue to be incorrectly considered running.
> > > > >
> > > > > I originally thought that this should not happen because such tra=
nsactions
> > > > > will be added to the builder's array of committed transactions by
> > > > > SnapBuildCommitTxn() anyway. However, I failed to notice that COM=
MIT record of
> > > > > a transaction listed in the xl_running_xacts WAL record is not gu=
aranteed to
> > > > > follow the xl_running_xacts record in WAL. In other words, even if
> > > > > xl_running_xacts is created before a COMMIT record of the contain=
ed
> > > > > transaction, it may end up at higher LSN in WAL. So the cleanup I=
 relied on
> > > > > might not take place.
> > > > >
> > > >
> > > > BTW, is it possible to write a test by using injection_points or via
> > > > manual steps (by using debugger, etc) so that we can more clearly
> > > > understand this problem and proposed fix?
> > >
> > > So far I could observe the situation in WAL, but have no idea how it =
can
> > > happen. For example, transaction 49242 gets committed here
> > >
> > > rmgr: Transaction len (rec/tot): 46/ 46, tx: 49242, lsn: 0/18BC28C8, =
prev
> > > 0/18BC2890, desc: COMMIT 2026-05-11 16:38:16.603265 CEST
> > >
> > > and then it appears in the 'xids' list of RUNNING_XACTS:
> > >
> > > rmgr: Standby     len (rec/tot):    106/   106, tx:          0, lsn:
> > > 0/18BC3140, prev 0/18BC3100, desc: RUNNING_XACTS nextXid 49255
> > > latestCompletedXid 49241 oldestRunningXid 49242; 13 xacts: 49248 4924=
9 49246
> > > 49243 49252 49251 49244 49245 49242 49250 49253 49254 49247; dbid:5
> > >
> > >
> > > I thought the situation is quite common (and therefore nothing of
> > > SnapBuildProcessRunningXacts() should be skipped), but when trying to
> > > reproduce the problem, I noticed that LogStandbySnapshot() shouldn't =
allow
> > > that ordering issue when logical decoding is enabled:
> > >
> > >       /*
> > >        * GetRunningTransactionData() acquired ProcArrayLock, we must =
release it.
> > >        * For Hot Standby this can be done before inserting the WAL re=
cord
> > >        * because ProcArrayApplyRecoveryInfo() rechecks the commit sta=
tus using
> > >        * the clog. For logical decoding, though, the lock can't be re=
leased
> > >        * early because the clog might be "in the future" from the POV=
 of the
> > >        * historic snapshot. This would allow for situations where we'=
re waiting
> > >        * for the end of a transaction listed in the xl_running_xacts =
record
> > >        * which, according to the WAL, has committed before the xl_run=
ning_xacts
> > >        * record. Fortunately this routine isn't executed frequently, =
and it's
> > >        * only a shared lock.
> > >        */
> > >       if (!logical_decoding_enabled)
> > >               LWLockRelease(ProcArrayLock);
> > >
> > > So I don't have the answer right now.
> >
> > I think now that "waiting for the end of a transaction listed in the
> > xl_running_xacts record" in the comment is about transaction removal fr=
om
> > procarray. However, the COMMIT record can still be ahead of xl_running_=
xacts
> > because RecordTransactionCommit() is called before
> > ProcArrayEndTransaction().
> >
>=20
> I see your point. Due to this, once the xmin regresses based on
> cluster-wide running_xact, some transaction could appear to be running
> when it should have appeared as committed.

The problem is that xmin does not advance when it should. Attached is a test
that reproduces the problem (it includes [1], to handle injection points in
background worker), I hope the comments in the specification file are helpf=
ul.

It's actually not exactly the problem reported in the stress test, but IMO =
the
core issue is the same: effects of some transactions are lost. In the stress
test, tuple deletion was lost, so the error was "could not create unique
index". Here I only demonstrate lost INSERT.

> Assuming, the problematic case is something
> like what I described, even than the fix of skipping cluster-wide
> running xacts and instead LOG db-specific running_xacts to help
> updating builder's xmin sounds inelegant and probably inefficient. For
> example, I think such a dependency means we can never enable
> db-specific snapshots on standby.

ok

[1] https://www.postgresql.org/message-id/4703.1774250534%40localhost

--=20
Antonin Houska
Web: https://www.cybertec-postgresql.com


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