Subject: Re: Protect syscache from bloating with negative cache
 entries
To: Tom Lane <tgl@sss.pgh.pa.us>
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CC: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>,
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From: Jim Nasby <Jim.Nasby@BlueTreble.com>
Message-ID: <276bcca0-9bfd-e60f-243d-baf081b41946@BlueTreble.com>
Date: Sun, 22 Jan 2017 16:41:18 -0600
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On 1/21/17 8:54 PM, Tom Lane wrote:
> Jim Nasby <Jim.Nasby@bluetreble.com> writes:
>> The other (possibly naive) question I have is how useful negative
>> entries really are? Will Postgres regularly incur negative lookups, or
>> will these only happen due to user activity?
> It varies depending on the particular syscache, but in at least some
> of them, negative cache entries are critical for performance.
> See for example RelnameGetRelid(), which basically does a RELNAMENSP
> cache lookup for each schema down the search path until it finds a
> match.

Ahh, I hadn't considered that. So one idea would be to only track 
negative entries on caches where we know they're actually useful. That 
might make the performance hit of some of the other ideas more 
tolerable. Presumably you're much less likely to pollute the namespace 
cache than some of the others.
-- 
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com
855-TREBLE2 (855-873-2532)


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