Re: Cost as used in 'EXPLAIN PLAN'

The Oracle Documentation (on-line version on your CD's) goes into detail on this topic....it's either in the Tuning Manual or the Application Developers Guide.

Craig

<raghuram_govind_at_my-deja.com> wrote in message news:7ltoc3$o60$1_at_nnrp1.deja.com...
> Hi Guys:
>
> I have been a oracle developer for some time and
> am just getting into optimization. The explain
> plan for a SQL statement is giving lots of info
> from which I am finding it difficult to understand
> what each one of those statements mean (physical
> reads, disk access, recursive calls, consistent
> gets ...). Can some one suggest a web site /
> document where I can find this info.
>
> In article <376a0732.6835168_at_newshost.us.oracle.com>,
> tkyte_at_us.oracle.com wrote:
> > A copy of this was sent to Greg Akins <insomnia_at_a1usa.net>
> > (if that email address didn't require changing)
> > On Thu, 17 Jun 1999 05:59:55 -0800, you wrote:
> >
> > >Hi,
> > >
> > >I understand that cost is a relative measure of query
> > >efficiency. Can somebody give me a rough idea of scale.
> > >
> > >For example, I do an explain plan and get some processes
> > >returned as Cost=1 or Cost=1 2. Then some get returned
> > >with Cost>5000.
> > >
> > >Is Cost>5000 an unreasonably large figure. Those costs are
> > >related to a sort that appears to stem from a 'select
> > >distinct' What can I do to minimize the cost and still
> > >effect the 'select distinct'
> > >
> > >-greg
> > >
> > >
> > >
> > >**** Posted from RemarQ - http://www.remarq.com - Discussions Start
> Here (tm) ****
> >
> > the costs within a query plan are useless to you. they mean nothing.
> ignore
> > them.
> >
> > the optimizer assigns costs to each phase of MANY different plans. It
> then
> > takes each plan and performs a function on the costs of each step. It
> then
> > picks the plan with the lowest cost.
> >
> > a query plan with the cost of 5,000,000 might very well run faster
> then a plan
> > with a cost of 1. You cannot compare the costs of 2 different plans
> ever -- it
> > doesn't make sense. only the optimizer can compare costs and then
> only for a
> > set of plans developed for the same query.
> >
> > the costs of certain steps are affected by many many things --
> statistics (or
> > lack thereof), heuristics, HINTS (thats how hints work -- by
> artifically
> > reducing the cost of certain steps in the hopes that makes the
> optimizer pick
> > the plan with that step in it), and so on...
> >
> > See http://www.oracle.com/ideveloper/ for my column 'Digging-in to
> Oracle8i'...
> > Mirrored (and more current) at http://govt.us.oracle.com/~tkyte/
> >
> > Current article is "Fine Grained Access Control", added June 8'th
> >
> > Thomas Kyte tkyte_at_us.oracle.com
> > Oracle Service Industries Reston, VA USA
> > --
> > Opinions are mine and do not necessarily reflect those of Oracle
> Corporation
> >
>
>
> Sent via Deja.com http://www.deja.com/
> Share what you know. Learn what you don't.
Received on Tue Jul 06 1999 - 18:24:46 CDT