Re: Asynchronous Append on postgres_fdw nodes.

On Wed, Mar 31, 2021 at 2:12 PM Etsuro Fujita <etsuro.fujita@gmail.com> wrote:
> On Wed, Mar 31, 2021 at 10:11 AM Kyotaro Horiguchi
> <horikyota.ntt@gmail.com> wrote:
> > +                * We'll prefer to consider this join async-capable if any table from
> > +                * either side of the join is considered async-capable.
> > +                */
> > +               fpinfo->async_capable = fpinfo_o->async_capable ||
> > +                       fpinfo_i->async_capable;
> >
> > We need to explain this behavior in the documentation.

> > It looks somewhat inconsistent to be inhibitive for the default value
> > of "async_capable", but agressive in merging?
>
> If the foreign table has async_capable=true, it actually means that
> there are resources (CPU, IO, network, etc.) to scan the foreign table
> concurrently.  And if any table from either side of the join has such
> resources, then they could also be used for the join.  So I don't
> think this behavior is aggressive.  I think it would be better to add
> more comments, though.
>
> I'll return to this after committing the patch.

I updated the above comment so that it explains the reason.  Please
find attached a patch.  I did some cleanup as well:

* Simplified code in ExecAppendAsyncEventWait() a little bit to avoid
duplicating the same nevents calculation, and updated comments there.

* Added an assertion to ExecAppendAsyncRequest().

* Updated comments for fetch_more_data_begin().

Best regards,
Etsuro Fujita