From: James Shaw <js102@zepler.net>
To: pgsql-docs@postgresql.org
Subject: Re: Possible mistake in 'Using EXPLAIN' section
Date: Mon, 22 Oct 2007 13:14:37 +0100
Message-ID: <c4a5ac250710220514l33976968l2eb819b29e6e7d07@mail.gmail.com> (raw)
In-Reply-To: <c4a5ac250710100551y51a93372u22e2eba612faf164@mail.gmail.com>
References: <c4a5ac250710100406j51c34512k47ef125c32c6f910@mail.gmail.com>
<13020.1192018254@sss.pgh.pa.us>
<c4a5ac250710100551y51a93372u22e2eba612faf164@mail.gmail.com>
On 10/10/2007, James Shaw <js102@zepler.net> wrote:
>
> 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.
Were there any thoughts on this? I was hoping this might be included in the
manual, since I felt it was somewhat clearer.
James
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To: pgsql-docs@postgresql.org
Cc: js102@zepler.net
Subject: Re: Possible mistake in 'Using EXPLAIN' section
In-Reply-To: <c4a5ac250710220514l33976968l2eb819b29e6e7d07@mail.gmail.com>
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