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From: Laurenz Albe <[email protected]>
To: sud <[email protected]>
Cc: pgsql-general <[email protected]>
Subject: Re: Long running query causing XID limit breach
Date: Thu, 23 May 2024 16:41:51 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAD=mzVVqR-mKUFHetsejFWSPQPbLjTVhCmBebJTFX5XmYp+nGg@mail.gmail.com>
References: <CAD=mzVXR3GjM0vcthMBwEdbOKqSKcv8oojSS9coczWRi9BRYTA@mail.gmail.com>
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On Thu, 2024-05-23 at 18:15 +0530, sud wrote:
> On Thu, May 23, 2024 at 1:45 PM Laurenz Albe <[email protected]> wrote:
> > If a long running query on the standby influences the primary, that means that
> > you have "hot_standby_feedback" set to "on".  Set it to "off".
> 
> Will the setting up of "hot_standby_feedback" value to OFF will cause the
> reader instance to give incorrect query results or unexpected query failure
> which will be potential inconsistency between the writer and reader instance,
> 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 , 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

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