RE: UncompBkts: what is it? (reference to 10053 trace)

As you may know, Oracle stores height balanced histograms in a "compressed" form, omitting consecutive rows with the same value at the end point (Oracle 8 erroneously reported the number of rows in the histogram table as num_buckets in the xx_tab_columns view). In the Oracle 10(.2 ?) 10053 trace, there are two cases for UncompBkts:
For height balanced histograms (Histogram: HtBal) UncompBkts is identical to buckets (#Bkts in the trace) and EndPtVals is the size of the "compressed" histogram, i.e. the number of rows in the histogram table for that column. From the difference you can gauge the number of popular values or the skew - could be many popular values or one hugely popular value. For frequency histograms (Histogram: Freq), UncompBkts is identical to the number of rows of the table and EndPtVals is identical to the number of buckets - and the number of distinct values of the column.

Quoting "Schultz, Charles" <sac_at_uillinois.edu>:

> Thanks, Mark and John, much appreciated!
>
> However, I still do not know what UncompBkts are. I have to infer that
> they have something to do with buckets. Uncompressed? If so, what does
> that mean? The numerical value is often close to the number of rows, but
> I do not know what the information implies.
>

-- 
regards

Wolfgang Breitling
Oracle 7,8,8i,9i OCP DBA
Centrex Consulting Corporation
www.centrexcc.com

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