Re: pg_stat_activity.waiting_start

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Joel Jacobson <joel(at)trustly(dot)com>
Cc: Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: pg_stat_activity.waiting_start
Date: 2016-12-24 02:00:59
Message-ID: 8779.1482544859@sss.pgh.pa.us
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Joel Jacobson <joel(at)trustly(dot)com> writes:
> We already have xact_start, query_start and backend_start
> to get the timestamptz for when different things happened.

> I would like to propose adding a fourth such column, "waiting_start",
> which would tell how long time a backend has been waiting.

The difficulty with that is it'd require a gettimeofday() call for
every wait start. Even on platforms where those are relatively cheap,
the overhead would be nasty --- and on some platforms, it'd be
astonishingly bad. We sweated quite a lot to get the overhead of
pg_stat_activity wait monitoring down to the point where it would be
tolerable for non-heavyweight locks, but I'm afraid this would push
it back into the not-tolerable range.

regards, tom lane

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