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.
Upcoming events at


-----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
Mladen Gogala
Oracle DBA


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