Subject: Re: Possible copy error in explain example

From: Michael Fuhr <[email-masked]>
Date: Mon, 11 Apr 2005 22:34:21 -0600

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/

---

From: Bruno Wolff III <[email-masked]>
Date: Mon, 11 Apr 2005 23:40:01 -0500

I was looking at:

---

From: Bruno Wolff III <[email-masked]>
Date: Tue, 12 Apr 2005 00:01:26 -0500

On Mon, Apr 11, 2005 at 22:34:21 -0600,
>   (try SHOW cpu_tuple_cost).

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.