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Re: Possible copy error in explain example

From: Bruno Wolff III <bruno(at)wolff(dot)to>
To: Michael Fuhr <mike(at)fuhr(dot)org>
Cc: pgsql-docs(at)postgresql(dot)org
Subject: Re: Possible copy error in explain example
Date: 2005-04-12 05:01:26
Message-ID: 20050412050126.GA32694@wolff.to (view raw or flat)
Thread:
Lists: pgsql-docs
On Mon, Apr 11, 2005 at 22:34:21 -0600,
  Michael Fuhr <mike(at)fuhr(dot)org> wrote:
> 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?

No, I was. I misread a comma as a period and when I scanned ahead to see
if I could find something similar in other examples I missed the part
where it added in the cpu cost. After I sent the message I looked some
more and spotted what I missed. I didn't have a copy back yet of the
original message, so my oops message wasn't threaded with the original.

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Subject: My oops (Was: possible copy error in explain docs)

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