Message-ID: <f8e40c2970ec116bef448f30e147091b880e5139.camel@cybertec.at>
Subject: Re: FDW connection drops with "Connection timed out" during async
 append query due to TCP receive buffer filling up
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: jiye <jiye_sw@126.com>, pgsql-bugs@lists.postgresql.org
Date: Tue, 10 Mar 2026 18:16:04 +0100
In-Reply-To: <10df46d9.7dc2.19cd740a640.Coremail.jiye_sw@126.com>
References: <10df46d9.7dc2.19cd740a640.Coremail.jiye_sw@126.com>
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On Tue, 2026-03-10 at 18:17 +0800, jiye wrote:
> I encountered an issue with foreign table asynchronous scans.
>=20
> I'm testing on PostgreSQL 17 in a Windows WSL environment, with TCP keepa=
live settings configured as follows:
> configure:
> > tcp_keepalives_idle =3D 2
> > tcp_keepalives_interval =3D 2
> > tcp_keepalives_count =3D 3
> > tcp_user_timeout =3D 9
> >=20
> The foreign table was created with:
> sql:
> > CREATE TABLE ftab (a int, data varchar);
> I inserted test data using:
> sql:
> > INSERT INTO ftab=20
> > SELECT generate_series(1, 10000),=20
> > =C2=A0 =C2=A0 =C2=A0 =C2=A0substring(md5(random()::text) || repeat(md5(=
random()::text), 127), 1, 40960);
> Then I executed an async append query that unions with a simple query:
> sql:
> > SELECT f1(), 'local-data'=20
> > UNION ALL=20
> > SELECT * FROM f_ftab=20
> > LIMIT 2;
> Note: f1() is a function that sleeps for 10 seconds, created by:
> sql
> > CREATE OR REPLACE FUNCTION f1()=20
> > RETURNS integer=20
> > LANGUAGE plpgsql AS $$
> > BEGIN=20
> > =C2=A0 =C2=A0 PERFORM pg_sleep(10);=20
> > =C2=A0 =C2=A0 RETURN 1;=20
> > END; $$;
> The abnormal behavior was reproduced: the foreign data server connection =
was disconnected, with the following error logs:
> text
> > 2026-03-10 17:52:55.969 CST [820] LOG: statement: FETCH 100 FROM c1
> > 2026-03-10 17:52:56.495 CST [820] LOG: could not receive data from clie=
nt: Connection timed out
> > 2026-03-10 17:52:56.495 CST [820] LOG: unexpected EOF on client connect=
ion with an open transaction

I am not a network expert, but if you set tcp_user_timeout =3D 9, why are y=
ou surprised if the
connection times out after 9 seconds?

Yours,
Laurenz Albe