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Date: Thu, 13 Dec 2012 17:36:26 -0500
From: Andrew Dunstan <andrew@dunslane.net>
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To: AI Rumman <rummandba@gmail.com>
CC: pgsql-performance@postgresql.org
Subject: Re: Why does the number of rows are different in actual and
 estimated.
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On 12/13/2012 05:12 PM, AI Rumman wrote:
> Why does the number of rows are different in actual and estimated?
>


Isn't that in the nature of estimates? An estimate is a heuristic guess 
at the number of rows it will find for the given query or part of a 
query. It's not uncommon for estimates to be out by several orders of 
magnitude. Guaranteeing estimates within bounded accuracy and in a given 
short amount of time (you don't want your planning time to overwhelm 
your execution time) isn't possible.

cheers

andrew


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