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In-Reply-To: <2d62f212-fce6-d639-b9eb-2a5bc4bec3b4@gmail.com>
From: Ashutosh Sharma <ashu.coek88@gmail.com>
Date: Thu, 12 Jan 2023 20:08:55 +0530
Message-ID: <CAE9k0PnRRdwRLnnYQkLK_dQrGM8yqmoyfa44NqUQHLr5gfJF_g@mail.gmail.com>
Subject: Re: Minimal logical decoding on standbys
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>, 
	Thomas Munro <thomas.munro@gmail.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, 
	Ibrar Ahmed <ibrar.ahmad@gmail.com>, Amit Khandekar <amitdkhan.pg@gmail.com>, fabriziomello@gmail.com, 
	tushar <tushar.ahuja@enterprisedb.com>, Rahila Syed <rahila.syed@2ndquadrant.com>, 
	pgsql-hackers <pgsql-hackers@postgresql.org>
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Hi,

On Thu, Jan 12, 2023 at 5:29 PM Drouvot, Bertrand
<bertranddrouvot.pg@gmail.com> wrote:
>
> Hi,
>
> On 1/11/23 7:04 PM, Drouvot, Bertrand wrote:
> > Hi,
> >
> > Please find V38 attached, I'll look at the other comments you've done in [1] on 0004 and 0006.
> >
> >

Sorry for joining late. I totally missed it. AFAICU, with this patch
users can now do LR from standby, previously they could only do it
from the primary server.

To start with, I have one small question:

I previously participated in the discussion on "Synchronizing the
logical replication slots from Primary to Standby" and one of the
purposes of that project was to synchronize logical slots from primary
to standby so that if failover occurs, it will not affect the logical
subscribers of the old primary much. Can someone help me understand
how we are going to solve this problem with this patch? Are we going
to encourage users to do LR from standby instead of primary to get rid
of such problems during failover?

Also, one small observation:

I just played around with the latest (v38) patch a bit and found that
when a new logical subscriber of standby is created, it actually
creates two logical replication slots for it on the standby server.
May I know the reason for creating an extra replication slot other
than the one created by create subscription command? See below:

Subscriber:
=========
create subscription t1_sub connection 'host=127.0.0.1 port=38500
dbname=postgres user=ashu' publication t1_pub;

Standby:
=======
postgres=# select * from pg_replication_slots;
                slot_name                |  plugin  | slot_type |
datoid | database | temporary | active | active_pid | xmin |
catalog_xmin | restart_lsn | confirmed_flush_lsn | wal_status | safe_w
al_size | two_phase
-----------------------------------------+----------+-----------+--------+----------+-----------+--------+------------+------+--------------+-------------+---------------------+------------+-------
--------+-----------
 pg_16399_sync_16392_7187728548042694423 | pgoutput | logical   |
5 | postgres | f         | t      |     112595 |      |          760 |
0/3082528   |                     | reserved   |
        | f
 t1_sub                                  | pgoutput | logical   |
5 | postgres | f         | t      |     111940 |      |          760 |
0/30824F0   | 0/3082528           | reserved   |
        | f

May I know the reason for creating pg_16399_sync_16392_7187728548042694423?

--
With Regards,
Ashutosh Sharma.