Обсуждение: Possible mistake in 'Using EXPLAIN' section

The example given has "Seq Scan on tenk1 (cost=0.00..458.00" in the example, but then says, "you will find out that tenk1 has 358 disk pages" and "the cost is estimated at 358 page reads".  Shouldn't this be 458 disk page reads?

I couldn't see this already discussed on this list.  Is there a URL that has the latest docs compiled from CVS?

Thanks
James Shaw

James Shaw a écrit :
> The example given has "Seq Scan on tenk1 (cost=0.00..458.00" in the
> example, but then says, "you will find out that |tenk1| has 358 disk
> pages" and "the cost is estimated at 358 page reads".  Shouldn't this be
> 458 disk page reads?
>
> I couldn't see this already discussed on this list.  Is there a URL that
> has the latest docs compiled from CVS?
>

http://momjian.us/main/writings/pgsql/sgml/

(built every five minutes)

Regards.


--
Guillaume.
 http://www.postgresqlfr.org
 http://dalibo.com

"James Shaw" <js102@zepler.net> writes:
> The example given has "Seq Scan on tenk1 (cost=0.00..458.00" in the example,
> but then says, "you will find out that tenk1 has 358 disk pages" and "the
> cost is estimated at 358 page reads".  Shouldn't this be 458 disk page
> reads?

No.  Why did you stop reading in mid-sentence?

            regards, tom lane

On 10/10/2007, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> "James Shaw" <js102@zepler.net> writes:
> > The example given has "Seq Scan on tenk1 (cost=0.00..458.00" in the example,
> > but then says, "you will find out that tenk1 has 358 disk pages" and "the
> > cost is estimated at 358 page reads".  Shouldn't this be 458 disk page
> > reads?
>
> No.  Why did you stop reading in mid-sentence?

I'm sorry, I misunderstood.  I've reread, and understand where the
extra 100 comes from now.  Perhaps this could be reworded:

This is about as straightforward as it gets.  If you do
SELECT relpages, reltuples FROM pg_class WHERE relname = 'tenk1';

you will find out that tenk1 has 358 disk pages and 10000 rows.  The
estimated cost is (disk pages read * seq_page_cost) + (number of rows
read * cpu_tuple_cost).  By default, seq_page_cost is 1.0 and
cpu_tuple_cost is 0.01.  Therefore, the estimated cost is (358 * 1.0)
+ (10000 * 0.01) = 458.

Thanks
James

On 10/10/2007, James Shaw <js102@zepler.net> wrote:

"James Shaw" <js102@zepler.net> writes:
>> I'm sorry, I misunderstood.  I've reread, and understand where the
>> extra 100 comes from now.  Perhaps this could be reworded:
>> ...

> Were there any thoughts on this?  I was hoping this might be included in the
> manual, since I felt it was somewhat clearer.

Done, sorry for not getting to it sooner.

            regards, tom lane