On Tue, May 11, 2021 at 6:27 PM Andrey Lepikhov <a.lepik...@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
