Date: Sat, 4 Apr 2020 11:20:15 +0200
From: Julien Rouhaud <rjuju123@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>,
	Kuntal Ghosh <kuntalghosh.2007@gmail.com>,
	Fujii Masao <masao.fujii@oss.nttdata.com>,
	Kirill Bychik <kirill.bychik@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
	Thomas Munro <thomas.munro@gmail.com>
Subject: Re: WAL usage calculation patch
Message-ID: <20200404092015.GE1206@nol>
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On Sat, Apr 04, 2020 at 02:39:32PM +0530, Amit Kapila wrote:
> On Sat, Apr 4, 2020 at 2:24 PM Julien Rouhaud <rjuju123@gmail.com> wrote:
> > >
> > > We can add if we want but I am not able to convince myself for that.
> > > Do you have any use case in mind?  I think in most of the cases
> > > (except for hint-bit WAL) it will be zero. If we are not sure of this
> > > we can also discuss it separately in a new thread once this
> > > patch-series is committed and see if anybody else sees the value of it
> > > and if so adding the code should be easy.
> >
> >
> > I'm mostly thinking of people trying to investigate possible slowdowns on a
> > hot-standby replica with a primary without wal_log_hints.  If they explicitly
> > ask for WAL information, we should provide them, even if it's quite unlikely to
> > happen.
> >
> 
> Yeah, possible but I am not completely sure.  I would like to hear the
> opinion of others if any before adding code for this.  How about if we
> first commit pg_stat_statements and wait for this till Monday and if
> nobody responds we can commit the current patch but would start a new
> thread and try to get the opinion of others?


I'm fine with it.


> 
> >
> > >
> > > >  I'm wondering how stable the normalized
> > > > WAL information would be in some regression tests, as the counters are only
> > > > showed if non zero.  Maybe it'd be better to remove them from the output, same
> > > > as the buffers?
> > > >
> > >
> > > Which regression tests are you referring to? pg_stat_statements?  If
> > > so, why would it be unstable?  It should always generate WAL although
> > > the exact values may differ and we have already taken care of that in
> > > the patch, no?
> >
> >
> > I'm talking about a hypothetical new EXPLAIN (ALAYZE, WAL) regression test,
> > which could be unstable for similar reason to why the first attempt to add
> > BUFFERS in the planning part of EXPLAIN was unstable.
> >
> 
> oh, then leave it for now because I don't see much use of those as the
> code path can anyway be hit by the tests added by pg_stat_statements
> patch.
> 


Perfect then!