From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Bruce Momjian <bruce@momjian.us>
Cc: Julien Rouhaud <rjuju123@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: torikoshia <torikoshia@oss.nttdata.com>
Cc: Atsushi Torikoshi <atorik@gmail.com>
Cc: Tatsuro Yamada <tatsuro.yamada.tf@nttcom.co.jp>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Evgeny Efimkin <efimkin@yandex-team.ru>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Feature improvement: can we add queryId for pg_catalog.pg_stat_activity view?
Date: Fri, 16 Oct 2020 13:03:55 -0300
Message-ID: <20201016160355.GA31474@alvherre.pgsql> (raw)
In-Reply-To: <20201016150449.GA23488@momjian.us>
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.)
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To: pgsql-hackers@postgresql.org
Cc: alvherre@alvh.no-ip.org, bruce@momjian.us, rjuju123@gmail.com, tgl@sss.pgh.pa.us, robertmhaas@gmail.com, michael@paquier.xyz, torikoshia@oss.nttdata.com, atorik@gmail.com, tatsuro.yamada.tf@nttcom.co.jp, tomas.vondra@2ndquadrant.com, efimkin@yandex-team.ru, pgsql-hackers@lists.postgresql.org
Subject: Re: Feature improvement: can we add queryId for pg_catalog.pg_stat_activity view?
In-Reply-To: <20201016160355.GA31474@alvherre.pgsql>
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