Date: Fri, 16 Oct 2020 13:03:55 -0300
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Bruce Momjian <bruce@momjian.us>
Cc: Julien Rouhaud <rjuju123@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>,
	Robert Haas <robertmhaas@gmail.com>,
	Michael Paquier <michael@paquier.xyz>,
	torikoshia <torikoshia@oss.nttdata.com>,
	Atsushi Torikoshi <atorik@gmail.com>,
	Tatsuro Yamada <tatsuro.yamada.tf@nttcom.co.jp>,
	Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	Evgeny Efimkin <efimkin@yandex-team.ru>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Feature improvement: can we add queryId for
 pg_catalog.pg_stat_activity view?
Message-ID: <20201016160355.GA31474@alvherre.pgsql>
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On 2020-Oct-16, Bruce Momjian wrote:

> On Thu, Oct 15, 2020 at 11:41:23AM +0800, Julien Rouhaud wrote:

> > I did some naive benchmarking.  Using a custom pgbench script with this query:

> > I can see around 2% overhead (this query is reported with ~ 3ms
> > latency average).  Adding a few joins, overhead goes down to 1%.
> 
> That number is too high to enable this by default.  I suggest we either
> improve the performance of this, or clearly document that you have to
> enable the hash computation to see the pg_stat_activity and
> log_line_prefix fields.

Agreed.  This is similar to how we used to deal with query strings: an
optional feature, disabled by default (cf.  commit b13c9686d084).

In this case, I suppose using pg_stat_statement would require to have it
enabled, and it'd just not collect anything if disabled.  Similarly, the
field would show NULL in pg_stat_activity or an empty string in
log_line_prefix/CSV logs.

So users that want it can easily have it, and users that don't are not
paying the price.

For maximum user-friendliness, pg_stat_statement could be loaded and
shmem-initialized even when query ID computation is turned off, and
you'd be able to enable query ID computation with just SIGHUP; so you
don't have to restart the server in order to enable statement tracking.
(I suppose we would forbid users from disabling query ID with SET,
though.)