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In-Reply-To: 
 <CAFiTN-sL7MVgbWh-_2YkC-dtHjZUEk8u1rBicM2+WQiFAUjKjw@mail.gmail.com>
From: vignesh C <vignesh21@gmail.com>
Date: Mon, 15 Jun 2026 10:46:07 +0530
Message-ID: 
 <CALDaNm24PqcBi8eOgxTTTHMVjMJdxFgU60Lw58XfYaaS4ni7yg@mail.gmail.com>
Subject: Re: Proposal: Conflict log history table for Logical Replication
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: shveta malik <shveta.malik@gmail.com>,
 Amit Kapila <amit.kapila16@gmail.com>,
	Nisha Moond <nisha.moond412@gmail.com>, Peter Smith <smithpb2250@gmail.com>,
	Masahiko Sawada <sawada.mshk@gmail.com>,
	Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Content-Type: text/plain; charset="UTF-8"
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Archived-At: 
 <https://www.postgresql.org/message-id/CALDaNm24PqcBi8eOgxTTTHMVjMJdxFgU60Lw58XfYaaS4ni7yg%40mail.gmail.com>
Precedence: bulk

On Sat, 13 Jun 2026 at 15:46, Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Thu, Jun 11, 2026 at 5:53=E2=80=AFPM vignesh C <vignesh21@gmail.com> w=
rote:
> >
> > On Thu, 11 Jun 2026 at 10:44, Dilip Kumar <dilipbalaut@gmail.com> wrote=
:
> > >
> > > Please find the rebased patch
> > > 1. It includes the new 0005 patch for reporting errors for DDLs on cl=
t.
> > >
> > > Open comments:
> > > 1. Recent comments from Nisha and Shveta after v47 are still open
> > > 2. Vignesh's patch for "describe related" changes needs a rebase. Can
> > > you do that, Vignesh? Meanwhile, I will close all the open comments
> > > and try to share a new version by EOD today.
> >
> > Here is the rebased version of the patch attached.
>
> Please find attached the latest patch. I have reordered the series,
> moving 0006 to 0002, and updated the lock restrictions. We now allow
> ACCESS SHARE mode exclusively to ensure pg_dump can acquire its
> necessary locks, while blocking higher-level locks to prevent
> interference with insertions into the conflict log tables.

I noticed that declaring a cursor with 'FOR UPDATE' on a conflict log
table currently fails:
postgres=3D*# DECLARE cur1 CURSOR FOR
    SELECT *
    FROM pg_conflict.pg_conflict_log_16404
    WHERE relid =3D 16402
    FOR UPDATE;
ERROR:  cannot lock rows in the conflict log table "pg_conflict_log_16404"

I'm not sure whether this restriction is intentional.

One benefit of supporting 'FOR UPDATE' cursors is that they provide
protection against concurrent modifications. For example:
**Session 1**
BEGIN;
DECLARE cur1 CURSOR FOR SELECT * FROM t3 WHERE c1 < 100 FOR UPDATE;
FETCH NEXT FROM cur1;

**Session 2**
DELETE FROM t3 WHERE c1 < 100;

In this case, the 'DELETE' in Session 2 will wait until Session 1
releases the row locks, preventing concurrent modification of the rows
being processed.

This can be useful for workflows that need to archive rows before
deleting them. For example:
DO $$
DECLARE
    r t3%ROWTYPE;
    cur1 CURSOR FOR SELECT * FROM t3 WHERE c1 < 100 FOR UPDATE;
BEGIN
    OPEN cur1
    LOOP
        FETCH cur1 INTO r;
        EXIT WHEN NOT FOUND;

        INSERT INTO t3_archive  VALUES (r.c1);

        DELETE FROM t3 WHERE CURRENT OF cur1;
    END LOOP;

    CLOSE cur1;
END $$;

Given these use cases, should 'FOR UPDATE' be allowed on conflict log
tables, or is the current behavior intentional for some reason that
I'm overlooking?

Regards,
Vignesh