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From: Julien Rouhaud <rjuju123@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: robertmhaas@gmail.com, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: pg_stat_statements issue with parallel maintenance (Was Re: WAL usage calculation patch)
Date: Sat, 28 Mar 2020 16:17:21 +0100
Message-ID: <20200328151721.GB12854@nol> (raw)
In-Reply-To: <20200328133827.GA12854@nol>
References: <5b71ad52-68c6-08cb-6408-31307e9e6f10@oss.nttdata.com>
	<20200319153129.GA36079@nol>
	<CAB-hujo_bnDsV7C1BNLMacC4gKTyHeR8_Fsdyv+rSoOcsk5HiA@mail.gmail.com>
	<a1452be6-9e82-59c4-3c54-b3f8919e75c0@oss.nttdata.com>
	<762d4ee8-9710-cdcd-e661-b5c79aaa9bf0@oss.nttdata.com>
	<CAOBaU_bb6gf-33qCsMiZYy6EupqAcT5xga3iM8q-=-Ap=+9w_w@mail.gmail.com>
	<CAB-hujrX8BWGYDH0h-4c0AQt0dfkNdaOPAaw5n7KWnzVe6vPOQ@mail.gmail.com>
	<CAOBaU_bRp3HmAnerZWFO00f8dx=nwU5WMUVY0woVY5kwMa_jEg@mail.gmail.com>
	<CAA4eK1+nLPguL+a1JStyx+L9xh0X3nQ3tXptorsggT6dn9oPLQ@mail.gmail.com>
	<20200328133827.GA12854@nol>

On Sat, Mar 28, 2020 at 02:38:27PM +0100, Julien Rouhaud wrote:
> On Sat, Mar 28, 2020 at 04:14:04PM +0530, Amit Kapila wrote:
> > 
> > I see some basic problems with the patch.  The way it tries to compute
> > WAL usage for parallel stuff doesn't seem right to me.  Can you share
> > or point me to any test done where we have computed WAL for parallel
> > operations like Parallel Vacuum or Parallel Create Index?
> 
> Ah, that's indeed a good point and AFAICT WAL records from parallel utility
> workers won't be accounted for.  That being said, I think that an argument
> could be made that proper infrastructure should have been added in the original
> parallel utility patches, as pg_stat_statement is already broken wrt. buffer
> usage in parallel utility, unless I'm missing something.

Just to be sure I did a quick test with pg_stat_statements behavior using
parallel/non-parallel CREATE INDEX and VACUUM, and unsurprisingly buffer usage
doesn't reflect parallel workers' activity.

I added an open for that, and adding Robert in Cc as 9da0cc352 is the first
commit adding parallel maintenance.





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