Date: Mon, 16 Feb 2026 10:10:21 +0000
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Michael Paquier <michael@paquier.xyz>, Jeff Davis <pgsql@j-davis.com>,
	Greg Sabino Mullane <htamfids@gmail.com>,
	pgsql-hackers@lists.postgresql.org
Subject: Re: Adding locks statistics
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Hi,

On Fri, Feb 13, 2026 at 04:13:39PM -0500, Andres Freund wrote:
> Hi,
> 
> On 2026-02-13 10:24:52 +0000, Bertrand Drouvot wrote:
> > 
> > That's fine by me. We could still add the others in the future if we feel the
> > need. Done that way in the attached.
> 
> I'm not sure that it's unproblematic to add multiple pgstat count calls to
> every lock acquisition, particularly if it's a fastpath acquisition or a
> virtualxid lock. Notably these are external function calls, not just
> increments of a counter in an inline function.

Yeah, we could create inline functions instead.

> I also don't really know what one would do with some of the information?
> 
> What does the number of virtualxid lock acquisitions tell you that the numbers
> of transactions doesn't already tell you in a more understandable way?

I agree not that much, except being able to compute a transactions/virtualxid
ratio or such. Also the idea was to report the same as pg_locks so that one
could start sampling pg_locks if he needs more details.

> What does it tell you that the deadlock checker ran N times? It notably
> doesn't count deadlocks, it counts how often we checked for deadlocks.
> 
> The percentage of fastpath locks also seems not really informative, because
> that could be because we ran out of space for fastpath locks, or because a
> lock mode that's ineligible for fastpath locks was used.

Right, maybe we could add an "exceed fastpath slots" or such counter?

> What I would actually count is the amount of time waiting for locks, that
> seems vastly more useful than the number of acquisitions.  We already do a
> GetCurrentTimestamp() inside the timer activations for deadlock timeout, we
> probably can figure out a way to reuse that to reduce the increase in overhead
> due to timing.  We could also just count the wait time after the deadlock
> check has run.

Yeah, providing the wait_time would be great. Just to be sure, are you suggesting
to remove all the fields (i.e requests, timeouts, deadlock_timeouts and fastpath)
and just add a wait_time field instead? I think that keeping requests would make
sense to be able to get the average wait time per request.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com