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Re: strange row number estimates in pg9.1rc1

From: "Sergey E(dot) Koposov" <math(at)sai(dot)msu(dot)ru>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org, Bruce Momjian <bruce(at)momjian(dot)us>
Subject: Re: strange row number estimates in pg9.1rc1
Date: 2011-08-30 16:34:12
Message-ID: alpine.LRH.2.00.1108302021220.21041@lnfm1.sai.msu.ru (view raw or flat)
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On Tue, 30 Aug 2011, Tom Lane wrote:

> Sure enough, that's the problem.  pg_upgrade leaves relpages/reltuples
> set to zero, but it also imports the visibility map pages from the old
> cluster.  If the old visibility map shows the table as all-visible,
> then this happens when you try to VACUUM ANALYZE the table:
>
> 1. VACUUM doesn't process any pages, so it has no tuple density
> estimate.  It leaves reltuples set to zero, but it does set relpages.
>
> 2. ANALYZE scans some part of the table.  It gets a tuple density
> estimate for those pages ... but if that's only a small fraction of
> the table, it believes the zero estimate of tuple density elsewhere.
> So you get only a small update of reltuples.

Thanks for figuring this out.
I wonder what should be the best way to proceed for already migrated 
databases -- running analyze repeatedly may not be the best way for 
very large clusters with large tables...

 	S

*******************************************************************
Sergey E. Koposov, PhD
Institute for Astronomy, Cambridge/Sternberg Astronomical Institute
Web: http://lnfm1.sai.msu.ru/~math
E-mail: math(at)sai(dot)msu(dot)ru

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