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From: Amit Kapila <amit.kapila16@gmail.com>
Date: Thu, 5 Sep 2024 11:09:26 +0530
Message-ID: <CAA4eK1KhAxe6=VTxa6xEbc22_VVBLsVuvRH8Wxws8i50gH=eqw@mail.gmail.com>
Subject: Re: Commit Timestamp and LSN Inversion issue
To: Aleksander Alekseev <aleksander@timescale.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>, shveta malik <shveta.malik@gmail.com>, 
	"Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, tomas@vondra.me
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On Wed, Sep 4, 2024 at 6:35=E2=80=AFPM Aleksander Alekseev
<aleksander@timescale.com> wrote:
>
> > > I don't think you can rely on a system clock for conflict resolution.
> > > In a corner case a DBA can move the clock forward or backward between
> > > recordings of Ts1 and Ts2. On top of that there is no guarantee that
> > > 2+ servers have synchronised clocks. It seems to me that what you are
> > > proposing will just hide the problem instead of solving it in the
> > > general case.
> > >
> >
> > It is possible that we can't rely on the system clock for conflict
> > resolution but that is not the specific point of this thread. As
> > mentioned in the subject of this thread, the problem is "Commit
> > Timestamp and LSN Inversion issue". The LSN value and timestamp for a
> > commit are not generated atomically, so two different transactions can
> > have them in different order.
>
> Hm.... Then I'm having difficulties understanding why this is a
> problem

This is a potential problem pointed out during discussion of CDR [1]
(Please read the point starting from "How is this going to deal .."
and response by Shveta). The point of this thread is that though it
appears to be a problem but practically there is no scenario where it
can impact even when we implement "last_write_wins" startegy as
explained in the initial email. If you or someone sees a problem due
to LSN<->timestamp inversion then we need to explore the solution for
it.

>
> and why it was necessary to mention CDR in this context in the
> first place.
>
> OK, let's forget about CDR completely. Who is affected by the current
> behavior and why would it be beneficial changing it?
>

We can't forget CDR completely as this could only be a potential
problem in that context. Right now, we don't have any built-in
resolution strategies, so this can't impact but if this is a problem
then we need to have a solution for it before considering a solution
like "last_write_wins" strategy.

Now, instead of discussing LSN<->timestamp inversion issue, you
started to discuss "last_write_wins" strategy itself which we have
discussed to some extent in the thread [2]. BTW, we are planning to
start a separate thread as well just to discuss the clock skew problem
w.r.t resolution strategies like "last_write_wins" strategy. So, we
can discuss clock skew in that thread and keep the focus of this
thread LSN<->timestamp inversion problem.

[1] - https://www.postgresql.org/message-id/CAJpy0uBWBEveM8LO2b7wNZ47raZ9tV=
Jw3D2_WXd8-b6LSqP6HA%40mail.gmail.com
[2] - https://www.postgresql.org/message-id/CAJpy0uD0-DpYVMtsxK5R%3DzszXauZ=
BayQMAYET9sWr_w0CNWXxQ%40mail.gmail.com

--=20
With Regards,
Amit Kapila.