RE: Incorrect cardinality estimate
- From: "Baumgartel, Paul" <paul.baumgartel@xxxxxxxxxxxxxxxxx>
- To: "'Alberto Dell'Era'" <alberto.dellera@xxxxxxxxx>
- Date: Wed, 24 Jan 2007 13:12:18 -0000
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
CREDIT SUISSE
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Phone 212.538.1143
paul.baumgartel@xxxxxxxxxxxxxxxxx
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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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