Re: How are SQL Profiles calculated?
- From: "Greg Rahn" <greg@xxxxxxxxxxxxxxxxxx>
- To: sacrophyte@xxxxxxxxx
- Date: Thu, 8 Jan 2009 09:36:11 -0800
On Thu, Jan 8, 2009 at 8:02 AM, Charles Schultz <sacrophyte@xxxxxxxxx> 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
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