From: Tom Lane <tgl@sss.pgh.pa.us>
To: Fujii Masao <masao.fujii@gmail.com>
cc: Jeremy Schneider <schneider@ardentperf.com>,
        Jacob Champion <jacob.champion@enterprisedb.com>,
        "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>,
        Marat Buharov <marat.buharov@gmail.com>,
        Greg Sabino Mullane <htamfids@gmail.com>,
        Thomas Munro <thomas.munro@gmail.com>
Subject: Re: client_connection_check_interval default value
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Comments: In-reply-to Fujii Masao <masao.fujii@gmail.com>
	message dated "Fri, 06 Feb 2026 08:42:42 +0900"
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Date: Thu, 05 Feb 2026 19:01:52 -0500
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Fujii Masao <masao.fujii@gmail.com> writes:
> On Fri, Feb 6, 2026 at 8:05 AM Jeremy Schneider
> <schneider@ardentperf.com> wrote:
>> I did see a real system outage that could have been prevented by an
>> appropriate default value here, since I didn't yet know to change it.

> I'm not sure that client_connection_check_interval needs to be enabled
> by default.

I think enabling it by default is a nonstarter, because it changes
behavior in a significant way.  Specifically, it's always been the
case that if the client disconnects during a non-SELECT query (or
anything that doesn't produce output), the backend would complete that
query before ending the session.  I think it's very likely that there
are users depending on that behavior.  Jeremy is describing an
application that evidently was built on the assumption that
disconnecting early would abort a wait, but that assumption was not
based on any testing.  I think it's good that we now have an option
to make such an assumption hold, but that does not translate to
"we should force that behavior on everybody".  Whether or not the
overhead is insignificant, there's a good chance that the change
would make more people unhappy than happy.

> The issue is that backends blocked in ProcSleep() are woken up every
> client_connection_check_interval and may emit a "still waiting" message
> each time if log_lock_waits is enabled. To mitigate this, just one idea is
> to add a flag to track whether the "still waiting" message has already been
> emitted during a call to ProcSleep(), and suppress further messages
> once it has been logged.

Independently of what's the default, it seems like it'd be valuable to
make that interaction better.  I think it is reasonable to keep on
emitting "still waiting" every so often, but we could probably
rate-limit that to a lot less than every 2 seconds.

			regards, tom lane