Re: Asynchronous Append on postgres_fdw nodes.

On Wed, Mar 31, 2021 at 6:55 PM Etsuro Fujita <etsuro.fujita@gmail.com> wrote:
> On Tue, Mar 30, 2021 at 8:40 PM Etsuro Fujita <etsuro.fujita@gmail.com> wrote:
> > I'm happy with the patch, so I'll commit it if there are no objections.
>
> Pushed.

I noticed that rescan of async Appends is broken when
do_exec_prune=false, leading to incorrect results on normal builds and
the following failure on assertion-enabled builds:

TRAP: FailedAssertion("node->as_valid_asyncplans == NULL", File:
"nodeAppend.c", Line: 1126, PID: 76644)

See a test case for this added in the attached.  The root cause would
be that we call classify_matching_subplans() to re-determine
sync/async subplans when called from the first ExecAppend() after the
first ReScan, even if do_exec_prune=false, which is incorrect because
in that case it is assumed to re-use sync/async subplans determined
during the the first ExecAppend() after Init.  The attached fixes this
issue.  (A previous patch also had this issue, so I fixed it, but I
think I broke this again when simplifying the patch :-(.)  I did a bit
of cleanup, and modified ExecReScanAppend() to initialize an async
state variable as_nasyncresults to zero, to be sure.  I think the
variable would have been set to zero before we get to that function,
so I don't think we really need to do so, though.

I will add this to the open items list for v14.

Best regards,
Etsuro Fujita