Date: Fri, 16 Oct 2020 11:04:49 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Julien Rouhaud <rjuju123@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>,
	Michael Paquier <michael@paquier.xyz>,
	Alvaro Herrera <alvherre@alvh.no-ip.org>,
	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: <20201016150449.GA23488@momjian.us>
References: <3250173.1602533250@sss.pgh.pa.us>
 <20201012205340.GD24889@momjian.us>
 <CAOBaU_ZbeQrUESCuLGk3sRZWxXFGaBBO39CxSsFxLeZGicUrJw@mail.gmail.com>
 <20201014140915.GJ24889@momjian.us>
 <CAOBaU_bMN1KAjn9Xb8Svhzf4CmiBo0OQioKRtxQs0_g9jFVEUg@mail.gmail.com>
 <20201014142523.GA9415@momjian.us>
 <3599578.1602685897@sss.pgh.pa.us>
 <CAOBaU_bGOT-6T4CSFc=VXviJXKYaUr2RXK_pRjVN38wMFj+ivg@mail.gmail.com>
 <20201014144050.GB9415@momjian.us>
 <CAOBaU_ZVmGPfKTwZ6cM_qdzaF2E1gMkrLDMwwLy4Z1JxQ6=CZg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: 
 <CAOBaU_ZVmGPfKTwZ6cM_qdzaF2E1gMkrLDMwwLy4Z1JxQ6=CZg@mail.gmail.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

On Thu, Oct 15, 2020 at 11:41:23AM +0800, Julien Rouhaud wrote:
> On Wed, Oct 14, 2020 at 10:40 PM Bruce Momjian <bruce@momjian.us> wrote:
> > There is that, and log_line_prefix, which I can imaging being useful.
> > My point is that if the queryid is visible, there should be a reason it
> > defaults to show empty.
> 
> I did some naive benchmarking.  Using a custom pgbench script with this query:
> 
> SELECT *
> FROM pg_class c
> JOIN pg_attribute a ON a.attrelid = c.oid
> ORDER BY 1 DESC
> LIMIT 1;
> 
> 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.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee