Re: [patch] BUG #15005: ANALYZE can make pg_class.reltuples inaccurate.

From: Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru>
To: David Gould <daveg(at)sonic(dot)net>
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org, Alina Alexeeva <alexeeva(at)adobe(dot)com>, Ullas Lakkur Raghavendra <lakkurra(at)adobe(dot)com>
Subject: Re: [patch] BUG #15005: ANALYZE can make pg_class.reltuples inaccurate.
Date: 2018-03-01 14:25:09
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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:
The Russian Postgres Company

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