Re: Possible copy error in explain example

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/