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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Michael Paquier <michael.paquier@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Cc: David Steele <david@pgmasters.net>
Cc: Jim Nasby <Jim.Nasby@bluetreble.com>
Cc: Craig Ringer <craig@2ndquadrant.com>
Cc: PostgreSQL mailing lists <pgsql-hackers@postgresql.org>
Subject: Re: Protect syscache from bloating with negative cache entries
Date: Wed, 29 Nov 2017 23:17:23 -0500
Message-ID: <84820.1512015443@sss.pgh.pa.us> (raw)
In-Reply-To: <CAB7nPqQvbMc5Exx=Qfx=42Osntqrm-thLLt3wCiLvLaokcsg4Q@mail.gmail.com>
References: <6eb3b24b-c0eb-b8ca-f847-609704e2a370@pgmasters.net>
	<3b7b7f90-db46-8c37-c4f7-443330c3ae33@pgmasters.net>
	<20170317.142313.232290068.horiguchi.kyotaro@lab.ntt.co.jp>
	<20171031.184622.92520047.horiguchi.kyotaro@lab.ntt.co.jp>
	<CAB7nPqSGdesvS-odcP+RRJaBOssPazwKURf1GAUYZED4dKSmiQ@mail.gmail.com>
	<CA+TgmobPHMmQEEUFDU6XCSj5ArEfyagS1+wisJ8QAY+EsUNbxg@mail.gmail.com>
	<CAB7nPqQvbMc5Exx=Qfx=42Osntqrm-thLLt3wCiLvLaokcsg4Q@mail.gmail.com>

Michael Paquier <michael.paquier@gmail.com> writes:
> On Thu, Nov 30, 2017 at 12:32 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>> I think we have to mark this as returned with feedback or rejected for
>> the reasons mentioned here:
>> http://postgr.es/m/CA+TgmoZjn28uYJRQ2K+5idhYxWBDER68sctoc2p_nW7h7JbhYw@mail.gmail.com

> Good point. I forgot this bit. Thanks for mentioning it I am switching
> the patch as returned with feedback.

We had a bug report just today that seemed to me to trace to relcache
bloat:
https://www.postgresql.org/message-id/flat/20171129100649.1473.73990%40wrigleys.postgresql.org

ISTM that there's definitely work to be done here, but as I said upthread,
I think we need a more holistic approach than just focusing on negative
catcache entries, or even just catcache entries.

The thing that makes me uncomfortable about this is that we used to have a
catcache size limitation mechanism, and ripped it out because it had too
much overhead (see commit 8b9bc234a).  I'm not sure how we can avoid that
problem within a fresh implementation.

			regards, tom lane




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