Message-ID: <891bcfec74f7358ef0212caf6565a35153dd2941.camel@cybertec.at>
Subject: Re: Long running query causing XID limit breach
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: sud <suds1434@gmail.com>
Cc: pgsql-general <pgsql-general@lists.postgresql.org>
Date: Thu, 23 May 2024 16:41:51 +0200
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On Thu, 2024-05-23 at 18:15 +0530, sud wrote:
> On Thu, May 23, 2024 at 1:45=E2=80=AFPM Laurenz Albe <laurenz.albe@cybert=
ec.at> wrote:
> > If a long running query on the standby influences the primary, that mea=
ns that
> > you have "hot_standby_feedback" set to "on".=C2=A0 Set it to "off".
>=20
> Will the setting up of "hot_standby_feedback" value to OFF will cause the
> reader instance to give incorrect=C2=A0query results or unexpected query =
failure
> which will be potential inconsistency between the writer and reader insta=
nce,
> as because those XID's can be removed/cleaned by the writer node even if =
its
> being read by the reader instance query. And it can have more replication=
 lag.

There will never be incorrect query results.

It can happen that a query on the standby gets canceled if you don't set
"max_standby_streaming_delay" to -1, but that can happen even if
"hot_standby_feedback" is "on".  It just happens less often.

The effect of setting "max_standby_streaming_delay" to -1 will often be a
replication delay if you run a long query.  That's what you have to
accept if you want to execute long-running queries.

You will never be able to have both of the following:
- queries never get canceled
- there is no replication delay

> So I'm wondering=C2=A0, if this setup is advisable one?

I'd say yes.  Anyway, if doesn't look like you have an alternative if
you want to run queries that take longer than it takes your transaction
ID counter to wrap around.

Yours,
Laurenz Albe