RE: Incorrect cardinality estimate

Quite correct.  I was not aware that a height-balanced histogram was of little 
use for queries citing unpopular values.  

A hint would work; per Wolfgang's suggestion, setting the density to a much 
lower value also yielded an efficient plan.  Thanks for everyone for their 
responses.

Paul Baumgartel
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-----Original Message-----
From: Alberto Dell'Era [mailto:alberto.dellera@xxxxxxxxx]
Sent: Tuesday, January 23, 2007 2:39 PM
To: paul.baumgartel@xxxxxxxxxxxxxxxxx
Cc: oracle-l@xxxxxxxxxxxxx
Subject: 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@xxxxxxxxxxxxxxxxx> 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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