Re: [HACKERS] Optimization for updating foreign tables in Postgres FDW - Mailing list pgsql-hackers

The subject of this thread is not directly related to this discussion
and we have a new thread [1] for relevant discussion. So, let's
discuss this further on that thread.


[1] https://www.postgresql.org/message-id/CAF1DzPU8Kx%2BfMXEbFoP289xtm3bz3t%2BZfxhmKavr98Bh-C0TqQ%40mail.gmail.com

On Tue, Apr 18, 2017 at 9:30 AM, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:
> At Tue, 18 Apr 2017 09:12:07 +0530, Ashutosh Bapat <ashutosh.bapat@enterprisedb.com> wrote in
<CAFjFpRcRjohUZaCNon1zkP7c6fo2qd5hnfHCT6t_39SGtS_oKQ@mail.gmail.com>
>> On Tue, Apr 18, 2017 at 8:12 AM, Kyotaro HORIGUCHI
>> <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
>> > At Mon, 17 Apr 2017 17:50:58 +0530, Ashutosh Bapat <ashutosh.bapat@enterprisedb.com> wrote in
<CAFjFpRdcWw4h0a-zrL-EiaekkPj8O0GR2M1FwZ1useSRfRm3-g@mail.gmail.com>
>> >> On Mon, Apr 17, 2017 at 1:53 PM, Kyotaro HORIGUCHI
>> >> <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
>> >> > At Thu, 13 Apr 2017 13:04:12 -0400, Robert Haas <robertmhaas@gmail.com> wrote in
<CA+TgmoaxnNmuONgP=bXJojrgbnMPTi6Ms8OSwZBC2YQ2ueUiSg@mail.gmail.com>
>> >> >> On Thu, Apr 21, 2016 at 10:53 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>> >> >> > On Thu, Apr 21, 2016 at 8:44 AM, Michael Paquier
>> >> >> > <michael.paquier@gmail.com> wrote:
>> >> >> >> On Thu, Apr 21, 2016 at 5:22 PM, Etsuro Fujita
>> >> >> >> <fujita.etsuro@lab.ntt.co.jp> wrote:
>> >> >> >>> Attached is an updated version of the patch, which modified Michael's
>> >> >> >>> version of the patch, as I proposed in [1] (see "Other changes:").  I
>> >> >> >>> modified comments for pgfdw_get_result/pgfdw_exec_query also, mainly because
>> >> >> >>> words like "non-blocking mode" there seems confusing (note that we have
>> >> >> >>> PQsetnonbloking).
>> >> >> >>
>> >> >> >> OK, so that is what I sent except that the comments mentioning PG_TRY
>> >> >> >> are moved to their correct places. That's fine for me. Thanks for
>> >> >> >> gathering everything in a single patch and correcting it.
>> >> >> >
>> >> >> > I have committed this patch.  Thanks for working on this.  Sorry for the delay.
>> >> >>
>> >> >> This 9.6-era patch, as it turns out, has a problem, which is that we
>> >> >> now respond to an interrupt by sending a cancel request and a
>> >> >> NON-interruptible ABORT TRANSACTION command to the remote side.  If
>> >> >> the reason that the user is trying to interrupt is that the network
>> >> >> connection has been cut, they interrupt the original query only to get
>> >> >> stuck in a non-interruptible wait for ABORT TRANSACTION.  That is
>> >> >> non-optimal.
>> >> >
>> >> > Agreed.
>> >> >
>> >> >> It is not exactly clear to me how to fix this.  Could we get by with
>> >> >> just slamming the remote connection shut, instead of sending an
>> >> >> explicit ABORT TRANSACTION?  The remote side ought to treat a
>> >> >> disconnect as equivalent to an ABORT anyway, I think, but maybe our
>> >> >> local state would get confused.  (I have not checked.)
>> >> >>
>> >> >> Thoughts?
>> >> >
>> >> > Perhaps we will get stuck at query cancellation before ABORT
>> >> > TRANSACTION in the case. A connection will be shut down when
>> >> > anything wrong (PQstatus(conn) != CONNECTION_OK and so) on
>> >> > aborting local transactoin . So I don't think fdw gets confused
>> >> > or sholdn't be confused by shutting down there.
>> >> >
>> >> > The most significant issue I can see is that the same thing
>> >> > happens in the case of graceful ABORT TRANSACTION. It could be a
>> >> > performance issue.
>> >> >
>> >> > We could set timeout here but maybe we can just slamming the
>> >> > connection down instead of sending a query cancellation. It is
>> >> > caused only by timeout or interrupts so I suppose it is not a
>> >> > problem *with a few connections*.
>> >> >
>> >> >
>> >> > Things are a bit diffent with hundreds of connections. The
>> >> > penalty of reconnection would be very high in the case.
>> >> >
>> >> > If we are not willing to pay such high penalty, maybe we are to
>> >> > manage busy-idle time of each connection and trying graceful
>> >> > abort if it is short enough, maybe having a shoft timeout.
>> >> >
>> >> > Furthermore, if most or all of the hundreds of connections get
>> >> > stuck, such timeout will accumulate up like a mountain...
>> >>
>> >> Even when the transaction is aborted because a user cancels a query,
>> >> we do want to preserve the connection, if possible, to avoid
>> >
>> > Yes.
>> >
>> >> reconnection. If the request to cancel the query itself fails, we
>> >> should certainly drop the connection. Here's the patch to do that.
>> >
>> > A problem I think on this would be that we still try to make
>> > another connection for canceling and it would stall for several
>> > minutes per connection on a packet stall, which should be done in
>> > a second on ordinary circumstances. Perhaps we might want here is
>> > async canceling with timeout.
>>
>> I am not sure what do you mean "make another connection for
>> cancelling.". Do you mean to say that PQcancel would make another
>> connection?
>
> Yes. It will take about 3 minutes on standard settings when no
> ACK returned. I thought that this discussion is based on such
> situation.
>
>> The patch proposed simply improves the condition when PQcancel has
>> returned a failure. Right now we ignore that failure and try to ABORT
>> the transaction, which is most probably going to get stalled or fail
>> to be dispatched. With the patch such a connection will be reset.
>
> Ah, I understand that. It is surely an improvement since it
> avoids useless ABORT TRANSACTION that is known to stall.
>
> regards,
>
> --
> Kyotaro Horiguchi
> NTT Open Source Software Center
>



-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company