Re: Asynchronous Append on postgres_fdw nodes.

On 3/6/21 14:49, Etsuro Fujita wrote:
> On Tue, May 11, 2021 at 6:55 PM Etsuro Fujita <etsuro.fujita@gmail.com> wrote:
>> On Tue, May 11, 2021 at 6:27 PM Andrey Lepikhov
>> <a.lepikhov@postgrespro.ru> wrote:
>>> On 11/5/21 12:24, Etsuro Fujita wrote:
>>
>>>>>       ->  Append (actual rows=3000 loops=1)
>>>>>             ->  Async Foreign Scan on f1 (actual rows=0 loops=1)
>>>>>             ->  Async Foreign Scan on f2 (actual rows=0 loops=1)
>>>>>             ->  Foreign Scan on f3 (actual rows=3000 loops=1)
>>>>>
>>>>> Here we give preference to the synchronous scan. Why?
>>>>
>>>> This would be expected behavior, and the reason is avoid performance
>>>> degradation; you might think it would be better to execute the async
>>>> Foreign Scan nodes more aggressively, but it would require
>>>> waiting/polling for file descriptor events many times, which is
>>>> expensive and might cause performance degradation.  I think there is
>>>> room for improvement, though.
>>> Yes, I agree with you. Maybe you can add note in documentation on
>>> async_capable, for example:
>>> "... Synchronous and asynchronous scanning strategies can be mixed by
>>> optimizer in one scan plan of a partitioned table or an 'UNION ALL'
>>> command. For performance reasons, synchronous scans executes before the
>>> first of async scan. ..."
>>
>> +1  But I think this is an independent issue, so I think it would be
>> better to address the issue separately.
> 
> I think that since postgres-fdw.sgml would be for users rather than
> developers, unlike fdwhandler.sgml, it would be better to explain this
> more in a not-too-technical way.  So how about something like this?
> 
> Asynchronous execution is applied even when an Append node contains
> subplan(s) executed synchronously as well as subplan(s) executed
> asynchronously.  In that case, if the asynchronous subplans are ones
> executed using postgres_fdw, tuples from the asynchronous subplans are
> not returned until after at least one synchronous subplan returns all
> tuples, as that subplan is executed while the asynchronous subplans
> are waiting for the results of queries sent to foreign servers.  This
> behavior might change in a future release.
Good, this text is clear for me.

-- 
regards,
Andrey Lepikhov
Postgres Professional