RE: Which plan is better - what COST really means ?
- From: Connor McDonald <hamcdc@xxxxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Sat, 15 Jan 2005 01:01:20 +0000 (GMT)
I can't speak for JL but basically I think he's basing cost as the optimizer's
estimate of IO's to
be performed.
--- Karen Morton <karen.morton@xxxxxxxxxx> wrote:
> Jonathan also mentioned to me that the cost estimate should be =
> multiplied by 'sreadtim'
> when running 9i with system statistics (cpu costing) enabled. The cost =
> multiplied by this
> value is the assumed time to complete.
>
> Cost is not the time estimate itself and this is what I was trying to =
> discern from
> Jonathan's earlier post. What I understand Jonathan was trying to say =
> was that cost is
> the value that should be multiplied by a time value estimate to =
> determine the estimated
> time to complete.
>
> The issue I still find for myself is that I rarely see this estimated =
> value be reliably
> close to the actual value for response time. Perhaps in the perfect =
> world, and I guess
> that's where the optimizer thinks it lives :), it would. But, even on a =
> test database
> where I am the only user executing a single query, I don't often see the =
> costed time
> estimate match the actual.
>
> I just wish the optimizer was perfect....then again, if that were the =
> case, many of us
> would have to find other ways to fill our time currently allotted to =
> query optimization.
> :)
>
>
>
> Karen Morton
> Hotsos Enterprises, Ltd.
> http://www.hotsos.com
> Upcoming events at http://www.hotsos.com/education/schedule.html
>
> =20
>
>
>
> -----Original Message-----
> From: Mladen Gogala [mailto:gogala@xxxxxxxxxxxxx]=20
> Sent: Friday, January 14, 2005 3:23 PM
> To: karen.morton@xxxxxxxxxx
> Cc: oracle-l@xxxxxxxxxxxxx
> Subject: Re: Which plan is better - what COST really means ?
>
>
>
> On 01/14/2005 05:43:53 PM, Karen Morton wrote:
>
> > In what time measurement is the cost? Seconds, centiseconds, =3D=20
> > microseconds? =3D20
>
> The measurements are in centiseconds, that is soft clock ticks. That is =
> one of the few
> things that wasn't converted to microseconds with =20
> oracle9i
> --=20
> Mladen Gogala
> Oracle DBA
>
>
> --
> //www.freelists.org/webpage/oracle-l
>
=====
Connor McDonald
Co-author: "Mastering Oracle PL/SQL - Practical Solutions"
ISBN: 1590592174
web: http://www.oracledba.co.uk
web: http://www.oaktable.net
email: connor_mcdonald@xxxxxxxxx
Coming Soon! "Oracle Insight - Tales of the OakTable"
"GIVE a man a fish and he will eat for a day. But TEACH him how to fish,
and...he will sit in a boat and drink beer all day"
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