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From: Etsuro Fujita <etsuro.fujita@gmail.com>
Date: Wed, 11 Mar 2026 17:01:02 +0900
Message-ID: <CAPmGK157wTfup6Jqt7K1p1=S0kGoBRSeQ1M50fLt93tcF1NQ5w@mail.gmail.com>
Subject: Re: Re: FDW connection drops with "Connection timed out" during async
 append query due to TCP receive buffer filling up
To: jiye <jiye_sw@126.com>
Cc: "David G. Johnston" <david.g.johnston@gmail.com>, 
	"pgsql-bugs@lists.postgresql.org" <pgsql-bugs@lists.postgresql.org>
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On Wed, Mar 11, 2026 at 3:25=E2=80=AFPM jiye <jiye_sw@126.com> wrote:
> Sorry, I made a mistake about the tcp_user_timeout configuration. Our app=
 sets it to 9000 (9 seconds), but it still errors out even with 9000 - it j=
ust takes a little longer to error.
> And about this point :
>   =3D> I don=E2=80=99t actually know whether or if =E2=80=9Cbuffer fillin=
g up=E2=80=9D is accurate or relevant here.  It doesn=E2=80=99t seem that w=
ay.  You haven=E2=80=99t demonstrated that scenario here, just a timeout be=
ing reached.
>   Actually i have caputured tcp dump firstly, and "tcp buffer filling up"=
 seem to be demonstrated by "TCP windows full" packet."
>   Secondly if data of fetch rows are not sufficiently wide, it does not r=
eproduce this issue.
>
> So i suspect that the reason for this connection timeout is that the tcp =
buffer is full.

I think this problem is not with async execution, but with your
environment; if the root cause of it is =E2=80=9CTCP windows full=E2=80=9D,=
 I think it
might fix it to 1) retrieve only needed columns from the remote server
and 2) decrease the fetch_size option for postgres_fdw.

Best regards,
Etsuro Fujita