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From: Etsuro Fujita <[email protected]>
To: jiye <[email protected]>
Cc: David G. Johnston <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Re: FDW connection drops with "Connection timed out" during async append query due to TCP receive buffer filling up
Date: Wed, 11 Mar 2026 17:01:02 +0900
Message-ID: <CAPmGK157wTfup6Jqt7K1p1=S0kGoBRSeQ1M50fLt93tcF1NQ5w@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<CAKFQuwYcEvfna4HUkG9VkX0cR_xbRRhT_0qDaAHhrJ4UX0EW-g@mail.gmail.com>
	<[email protected]>

On Wed, Mar 11, 2026 at 3:25 PM jiye <[email protected]> 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 just takes a little longer to error.
> And about this point :
>   => I don’t actually know whether or if “buffer filling up” is accurate or relevant here.  It doesn’t seem that way.  You haven’t demonstrated that scenario here, just a timeout being 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 reproduce 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 “TCP windows full”, 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

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