Re: Long running query causing XID limit breach

Hi,

I am following this very interesting thread.

From the documentation
https://www.postgresql.org/docs/current/runtime-config-client.html#GUC-IDLE-IN-TRANSACTION-SESSION-TIMEOUT,
the 0 value will disable the timeout (not -1).

On Wed, Jun 5, 2024 at 8:25 AM sud <suds1434(at)gmail(dot)com> wrote:

> Hello Laurenz,
>
> Thank you so much.This information was really helpful for us
> understanding the working of these parameters.
>
> One follow up question i have , as we are setting one of the
> standby/replica with value idle_in_transaction_session_timeout=-1 which can
> cause the WAL's to be heavily backlogged in a scenario where we have a
> query running for very long time on that instance. So in that case will
> there be chances of instance restart and if that can be avoided anyway?
>
> And the plan is to set these system parameters with different values in
> writer/read replica , so in that case if we apply the "alter system"
> command on the primary , won't the WAL going to apply those same commands
> forcibly on reader instance making those same as the writer instance
> configuration( but we want the reader replica configuration to be different
> from writer)?
>
> Appreciate your guidance.
>
> On Wed, May 29, 2024 at 1:38 PM Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
> wrote:
>
>> On Wed, 2024-05-29 at 01:34 +0530, yudhi s wrote:
>> > > The only way you can have no delay in replication AND no canceled
>> queries is
>> > > if you use two different standby servers with different settings for
>> > > "max_standby_streaming_delay". One of the server is for HA, the
>> other for
>> > > your long-running queries.
>> >
>> > When you suggest having different max_standby_streaming_delay for first
>> replica
>> > (say 10 sec for High availability) and second replica(say -1 for long
>> running queries).
>> > Do you also suggest keeping "hot_feedback_standby" as "OFF" for all
>> the three
>> > instances i.e. master and both the replicas?
>>
>> The parameter is ignored on the master.
>> It needs to be off on the standby that is running long queries.
>> For the other standby it probably doesn't matter if you are not running
>> any
>> queries on it. I would leave "hot_standby_feedback = off" there as well.
>>
>> Actually, I would set "hot_standby = off" on the standby that is only used
>> for HA.
>>
>>
>> - I would leave "hot_standby_feedback" off everywhere.
>> - "max_standby_streaming_delay" should be -1 on the reporting standby and
>> very
>> low or 0 on the HA standby. It doesn't matter on the primary.
>> - "statement_timeout" should be way lower on the first two nodes.
>> - "idle_in_transaction_session_timeout" is good.
>> - I would leave "autovacuum_freeze_max_age" at the default setting but
>> 100 million
>> is ok too.
>>
>> Yours,
>> Laurenz Albe
>>
>