Re: Incorrect cardinality estimate

Given the data distribution, it's unlikely (impossible) that the value "12/27/2006 00:00:00"
shows up as *popular* value in the histogram, in which case the CBO will compute the cardinality as density * num_rows (check Jonathan's "Cost Based Oracle", page 172; I
remember a paper by Wolfgang saying the same, check his site).

Check the density of ODS_PROCESS_DATE, I guess that it will be

.006292114 = 1223485 / 194447369

Probably you'll need a good old hint ...

On 1/23/07, Baumgartel, Paul <paul.baumgartel_at_credit-suisse.com> wrote:
> Following up on my post from Friday regarding an optimizer plan to use an index join rather than a simple index lookup.
>
> Wolfgang Breitling pointed out that the optimizer estimated that the query would return 1.3M rows. In fact the query returns 492 rows, so I took at look at data value distribution in the predicate column (query is select trans_id from ods_execution where ods_process_date='12/27/2006'). Distribution of ods_process_date is definitely skewed (here are the last few rows of the count of each value):
>
> ODS_PROCESS_DATE COUNT(*)
> ------------------- -------
> /16/2006 00:00:00 1544886
> 11/16/2006 08:53:52 1
> 11/17/2006 00:00:00 1226408
> 11/17/2006 15:50:12 1
> 11/17/2006 16:45:10 1
> 11/17/2006 16:46:00 1
> 11/17/2006 16:46:10 1
> 11/20/2006 00:00:00 12
> 12/27/2006 00:00:00 492
>
> So, I have created a histogram on the ods_process_date column, via
>
> exec dbms_stats.gather_table_stats('ODS','ODS_EXECUTION',-
> > method_opt=>'FOR COLUMNS ODS_PROCESS_DATE SIZE 254',-
> > stattab=>'ODS_STATS',statown=>'P_BAUMGA2',statid=>'NoHisto')
>
> Now the puzzle: after flushing the shared pool (to force a re-parse), the optimizer's cardinality estimate, and thus its plan, is not changing. A 10053 trace shows that the optimizer is aware of the histogram:
>
> SINGLE TABLE ACCESS PATH
> Column: ODS_PROCES Col#: 14 Table: ODS_EXECUTION Alias: ODS_EXECUTION
> NDV: 772 NULLS: 0 DENS: 6.2921e-03
> HEIGHT BALANCED HISTOGRAM: #BKT: 254 #VAL: 143
> TABLE: ODS_EXECUTION ORIG CDN: 194447369 ROUNDED CDN: 1223485 CMPTD CDN: 1223485

-- 
Alberto Dell'Era
"Per aspera ad astra"
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