Subject: Re: [patch] BUG #15005: ANALYZE can make pg_class.reltuples
 inaccurate.
To: David Gould <daveg@sonic.net>
Cc: pgsql-hackers@lists.postgresql.org, Alina Alexeeva <alexeeva@adobe.com>,
 Ullas Lakkur Raghavendra <lakkurra@adobe.com>
References: <20180117164916.3fdcf2e9@engels>
 <fa8122c2-ea7a-ff38-49ac-e6616aec38ea@postgrespro.ru>
 <20180228192337.67cc3f99@engels>
From: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>
Message-ID: <4988c54a-1709-a978-9a43-65f8a1e09e94@postgrespro.ru>
Date: Thu, 1 Mar 2018 17:25:09 +0300
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.5.0
MIME-Version: 1.0
In-Reply-To: <20180228192337.67cc3f99@engels>
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Precedence: bulk

On 01.03.2018 06:23, David Gould wrote:
> In theory the sample pages analyze uses should represent the whole table
> fairly well. We rely on this to generate pg_statistic and it is a key
> input to the planner. Why should we not believe in it as much only for
> reltuples? If the analyze sampling does not work, the fix would be to improve
> that, not to disregard it piecemeal.

Well, that sounds reasonable. But the problem with the moving average 
calculation remains. Suppose you run vacuum and not analyze. If the 
updates are random enough, vacuum won't be able to reclaim all the 
pages, so the number of pages will grow. Again, we'll have the same 
thing where the number of pages grows, the real number of live tuples 
stays constant, and the estimated reltuples grows after each vacuum run.

I did some more calculations on paper to try to understand this. If we 
average reltuples directly, instead of averaging tuple density, it 
converges like it should. The error with this density calculation seems 
to be that we're effectively multiplying the old density by the new 
number of pages. I'm not sure why we even work with tuple density. We 
could just estimate the number of tuples based on analyze/vacuum, and 
then apply moving average to it. The calculations would be shorter, too. 
What do you think?

-- 
Alexander Kuzmenkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company