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Re: Possible mistake in 'Using EXPLAIN' section

From: "James Shaw" <js102(at)zepler(dot)net>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-docs(at)postgresql(dot)org
Subject: Re: Possible mistake in 'Using EXPLAIN' section
Date: 2007-10-10 12:51:21
Message-ID: c4a5ac250710100551y51a93372u22e2eba612faf164@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-docs
On 10/10/2007, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> "James Shaw" <js102(at)zepler(dot)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

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