Re: Protect syscache from bloating with negative cache entries

From: Jim Nasby <Jim(dot)Nasby(at)BlueTreble(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, <robertmhaas(at)gmail(dot)com>, <craig(at)2ndquadrant(dot)com>, <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Protect syscache from bloating with negative cache entries
Date: 2017-01-22 22:51:53
Message-ID: 683113d7-c7a7-1a0f-da7c-b197b08a6440@BlueTreble.com
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On 1/22/17 4:41 PM, Jim Nasby wrote:
> On 1/21/17 8:54 PM, Tom Lane wrote:
>> Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)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.

Ok, after reading the code I see I only partly understood what you were
saying. In any case, it might still be useful to do some testing with
CATCACHE_STATS defined to see if there's caches that don't accumulate a
lot of negative entries.

Attached is a patch that tries to document some of this.
--
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)

Attachment Content-Type Size
negative.patch text/plain 996 bytes

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