Re: Re: Density statistic calculation in case of histogram
- From: Neeraj Bhatia <neeraj.dba@xxxxxxxxx>
- To: "Alberto Dell'Era" <alberto.dellera@xxxxxxxxx>
- Date: Thu, 9 Apr 2009 07:55:53 +0530
Thanks Alberto! It was w good read (11g new density).
One question: You have mentioned that CBO classifies at run-time the
histogram type as it's stored in DD using a heuristic. As per my knowledge,
based on NDV and #Buckets requested, CBO decides whether to create HB or
Freq histogram and save the type in data dictionary, which is visible
through dba_tab_columns.histogram column.
Please correct me!
On Thu, Apr 9, 2009 at 3:44 AM, Alberto Dell'Era
<alberto.dellera@xxxxxxxxx>wrote:
> On Mon, Apr 6, 2009 at 17:20, Neeraj Bhatia <neeraj.dba@xxxxxxxxx> wrote:
> > Basically I am interested to know how density is calculated in case of
> > height-based histograms. Also, how CBO calculate selectivity.
>
> I wouldn't investigate too much the density in 10.x since the formula
> has changed (to a very intuitive one) in 11gR1:
>
> http://www.adellera.it/investigations/11g_newdensity/index.html
>
> so, the 10.x formula has been "deprecated" by the kernel developers.
>
> I did find a statistical justification for the 10.x computation, but
> I have never "published" it for the above reasons, it is no longer
> important.
>
> hth
> alberto
>
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