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From: Gary Doades <gpd@gpdnet.co.uk>
To: pgsql-performance@postgresql.org
Subject: Re: Optimizer showing wrong rows in plan
Date: Sun, 28 Mar 2010 10:27:22 +0100
Message-ID: <4BAF20FA.30800@gpdnet.co.uk> (raw)
In-Reply-To: <645d9d71003280207p14fa4628i91b51ca8ee954216@mail.gmail.com>
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On 28/03/2010 10:07 AM, Tadipathri Raghu wrote:
> Hi All,
> I want to give some more light on this by analysing more like this
> 1. In my example I have created a table with one column as INT( which 
> occupies 4 bytes)
> 2. Initially it occupies one page of  space on the file that is (8kb).
> So, here is it assuming these many rows may fit in this page. Clarify 
> me on this Please.

Like I said, it's just a guess. With no statistics all postgres can do 
is guess, or in this case use the in-built default for a newly created 
table. It could guess 1 or it could guess 10,000,000. What it does is 
produce a reasonable guess in the absence of any other information.

You should read the postgres documentation for further information about 
statistics and how the optimizer uses them.


Regards,
Gary.





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