Re: How are SQL Profiles calculated?
From: Greg Rahn <greg_at_structureddata.org>
Date: Thu, 8 Jan 2009 09:36:11 -0800
Message-ID: <a9c093440901080936n66dfe2bdgc859cc861efb40e0_at_mail.gmail.com>
On Thu, Jan 8, 2009 at 8:02 AM, Charles Schultz <sacrophyte_at_gmail.com> wrote:
> How does Oracle come up with those scale numbers?
Date: Thu, 8 Jan 2009 09:36:11 -0800
Message-ID: <a9c093440901080936n66dfe2bdgc859cc861efb40e0_at_mail.gmail.com>
On Thu, Jan 8, 2009 at 8:02 AM, Charles Schultz <sacrophyte_at_gmail.com> wrote:
> How does Oracle come up with those scale numbers?
Basically by using the estimated vs. actual cardinality numbers.
> How are those scale numbers more significant than gathering a complete statistical picture?
They are generally more accurate as they are a result of executing portions of the full plan to get the actual cardinality. The optimizer estimates (forecasts) the number of rows based on what data points it knows, but dbms_sqltune is using hindsight. SCALE_ROWS is basically the "fudge factor" to "reconcile" the optimizer's estimate to result in the actual row source cardinality.
Hope that helps.
-- Regards, Greg Rahn http://structureddata.org -- http://www.freelists.org/webpage/oracle-lReceived on Thu Jan 08 2009 - 11:36:11 CST