Re: Re: FDW connection drops with "Connection timed out" during async append query due to TCP receive buffer filling up

On Wed, Mar 11, 2026 at 3:25 PM jiye <jiye_sw(at)126(dot)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 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