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Re: reltuples after vacuum and analyze

From: Litao Wu <litaowu(at)yahoo(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: reltuples after vacuum and analyze
Date: 2005-01-25 18:29:26
Message-ID: 20050125182926.98344.qmail@web30309.mail.mud.yahoo.com (view raw or flat)
Thread:
Lists: pgsql-performance
Thanks,

Then how to explain relpages 
(size_kb in result returned)?

SELECT relname, relpages * 8 as size_kb,
relfilenode, reltuples
FROM pg_class c1
WHERE relkind = 'r'
AND relname = 'my_tab';
     relname      | size_kb | relfilenode | reltuples
------------------+---------+-------------+-----------
 my_tab           |   30088 |   266181583 |    165724
analyze my_tab;
     relname      | size_kb | relfilenode |  reltuples
------------------+---------+-------------+-------------
 my_tab           | 2023024 |   266181583 |
1.12323e+07
vacuum my_tab;
SELECT relname, relpages * 8 as size_kb,
relfilenode, reltuples
FROM pg_class c1
WHERE relkind = 'r'
AND relname = 'my_tab';
     relname      | size_kb | relfilenode | reltuples
------------------+---------+-------------+-----------
 my_tab           | 2038016 |   266181583 |    189165
(1 row)

--- Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Litao Wu <litaowu(at)yahoo(dot)com> writes:
> > I noticed that reltuples are way off if
> > I vacuum the table and analyze the table.
> > And the data (296901) after vacuum seems 
> > accurate while
> > the reltuples (1.90744e+06)
> > after anlayze is too wrong.
> 
> VACUUM derives an exact count because it scans the
> whole table.  ANALYZE
> samples just a subset of the table and extrapolates.
>  It would appear
> that you've got radically different tuple densities
> in different parts
> of the table, and that's confusing ANALYZE.
> 
> > My PG version is 7.3.2 (I know it is old).
> 
> 8.0's ANALYZE uses a new sampling method that we
> think is less prone
> to this error, though of course any sampling method
> will fail some of
> the time.
> 
> 			regards, tom lane
> 



		
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