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

On Tue, Oct 31, 2017 at 6:17 PM, Alexander Korotkov
<a(dot)korotkov(at)postgrespro(dot)ru> wrote:
> On Tue, Oct 31, 2017 at 5:16 AM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
> wrote:
>>
>> On Mon, Oct 30, 2017 at 10:16 PM, Robert Haas <robertmhaas(at)gmail(dot)com>
>> wrote:
>> > On Tue, Oct 24, 2017 at 1:26 PM, Ivan Kartyshov
>> > <i(dot)kartyshov(at)postgrespro(dot)ru> wrote:
>> >> Hello. I made some bugfixes and rewrite the patch.
>> >
>> > I don't think it's a good idea to deliberately leave the state of the
>> > standby different from the state of the master on the theory that it
>> > won't matter. I feel like that's something that's likely to come back
>> > to bite us.
>>
>> I agree with Robert. What happen if we intentionally don't apply the
>> truncation WAL and switched over? If we insert a tuple on the new
>> master server to a block that has been truncated on the old master,
>> the WAL apply on the new standby will fail? I guess there are such
>> corner cases causing failures of WAL replay after switch-over.
>
>
> Yes, that looks dangerous. One approach to cope that could be teaching heap
> redo function to handle such these situations. But I expect this approach
> to be criticized for bad design. And I understand fairness of this
> criticism.
>
> However, from user prospective of view, current behavior of
> hot_standby_feedback is just broken, because it both increases bloat and
> doesn't guarantee that read-only query on standby wouldn't be cancelled
> because of vacuum. Therefore, we should be looking for solution: if one
> approach isn't good enough, then we should look for another approach.
>
> I can propose following alternative approach: teach read-only queries on hot
> standby to tolerate concurrent relation truncation. Therefore, when
> non-existent heap page is accessed on hot standby, we can know that it was
> deleted by concurrent truncation and should be assumed to be empty. Any
> thoughts?
>

You also meant that the applying WAL for AccessExclusiveLock is always
skipped on standby servers to allow scans to access the relation?

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center