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Date: Mon, 11 Apr 2005 22:34:21 -0600
From: Michael Fuhr <mike@fuhr.org>
To: Bruno Wolff III <bruno@wolff.to>, pgsql-docs@postgresql.org
Subject: Re: Possible copy error in explain example
Message-ID: <20050412043421.GA60302@winnie.fuhr.org>
References: <20050412044001.GA32308@wolff.to>
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On Mon, Apr 11, 2005 at 11:40:01PM -0500, Bruno Wolff III wrote:
> I was looking at:
> http://candle.pha.pa.us/main/writings/pgsql/sgml/performance-tips.html
> 
> And noticed in the first example that it is claimed the cost estimate
> is 233 disk block reads. However, "233" doesn't appear in the explain
> output shown. I think that "233" is supposed to match the "333" in
> the explain output, but I am not 100% sure.

This part?

  This is about as straightforward as it gets.  If you do

    SELECT * FROM pg_class WHERE relname = 'tenk1';

  you will find out that tenk1 has 233 disk pages and 10000 rows.
  So the cost is estimated at 233 page reads, defined as costing
  1.0 apiece, plus 10000 * cpu_tuple_cost which is currently 0.01
  (try SHOW cpu_tuple_cost).

Doesn't that work out to

  (233 * 1.0) + (10000 * 0.01) = 233.0 + 100.0 = 333.0

or am I missing something?

-- 
Michael Fuhr
http://www.fuhr.org/~mfuhr/