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.
Best regards,
Etsuro Fujita