RE: Which plan is better - what COST really means ?
- From: "Karen Morton" <karen.morton@xxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Fri, 14 Jan 2005 15:50:47 -0800
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
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