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Re: Possible copy error in explain example

From: Michael Fuhr <mike(at)fuhr(dot)org>
To: Bruno Wolff III <bruno(at)wolff(dot)to>, pgsql-docs(at)postgresql(dot)org
Subject: Re: Possible copy error in explain example
Date: 2005-04-12 04:34:21
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Lists: pgsql-docs
On Mon, Apr 11, 2005 at 11:40:01PM -0500, Bruno Wolff III wrote:
> I was looking at:
> 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

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