Re: Asynchronous Append on postgres_fdw nodes.

On Tue, Apr 27, 2021 at 3:57 PM Andrey V. Lepikhov
<a.lepikhov@postgrespro.ru> wrote:
> One more question. Append choose async plans at the stage of the Append
> plan creation.
> Later, the planner performs some optimizations, such as eliminating
> trivial Subquery nodes. So, AsyncAppend is impossible in some
> situations, for example:
>
> (SELECT * FROM f1 WHERE a < 10)
>    UNION ALL
> (SELECT * FROM f2 WHERE a < 10);
>
> But works for the query:
>
> SELECT *
>    FROM (SELECT * FROM f1 UNION ALL SELECT * FROM f2) AS q1
> WHERE a < 10;
>
> As far as I understand, this is not a hard limit.

I think so, but IMO I think this would be an improvement rather than a bug fix.

> We can choose async
> subplans at the beginning of the execution stage.
> For a demo, I prepared the patch (see in attachment).
> It solves the problem and passes the regression tests.

Thanks for the patch!  IIUC, another approach to this would be the
patch you proposed before [1].  Right?

I didn't have time to look at the patch in [1] for PG14.  My apologies
for that.  Actually, I was planning to return it when the development
for PG15 starts.

Sorry for the late reply.

Best regards,
Etsuro Fujita

[1] https://www.postgresql.org/message-id/7fe10f95-ac6c-c81d-a9d3-227493eb9055%40postgrespro.ru