Re: WIP: long transactions on hot standby feedback replica / proof of concept

On Mon, Sep 4, 2017 at 4:34 PM, <i(dot)kartyshov(at)postgrespro(dot)ru> wrote:
> Our clients complain about this issue and therefore I want to raise the
> discussion and suggest several solutions to this problem:
>
> I. Why does PG use Fatal when Error is enough to release lock that rises
> lock conflict?
> "If (RecoveryConflictPending && DoingCommandRead)"
>
> II. Do we really need to truncate the table on hot standby exactly at the
> same time when truncate on master occurs?
>
> In my case conflict happens when the autovacuum truncates table tbl1 on
> master while backend on replica is performing a long transaction involving
> the same table tbl1. This happens because truncate takes an
> AccessExclusiveLock. To tackle this issue we have several options:
>
> 1. We can postpone the truncate on the master until all the replicas have
> finished their transactions (in this case, feedback requests to the master
> should be sent frequently)
> Patch 1
> vacuum_lazy_truncate.patch
>
> 2. Maybe there is an option somehow not to send AccessExclusiveLock and not
> to truncate table on the replica right away. We could try to wait a little
> and truncate tbl1 on replica again.
>

Can max_standby_streaming_delay help in this situation (point number - 2)?

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com