Re: Very Strange Query Access Plan
- From: "Terry Sutton" <terrysutton@xxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Wed, 3 Oct 2007 17:59:16 -0700
Tom,
You might also want to check the num_distinct column of user_tab_columns to see
if that's a realistic number. I've had situations where it was really low
(like 400 when there were millions of distinct values) which resulted from too
small an estimate using analyze or dbms_stats.
--Terry
----- Original Message -----
From: Mercadante, Thomas F (LABOR)
To: oracle-l@xxxxxxxxxxxxx
Sent: Tuesday, October 02, 2007 11:29 AM
Subject: RE: Very Strange Query Access Plan
All,
Thanks to Alvaro Jose Fernandez & Ric Van Dyke, this is solved. The DENSITY
and CLUSTER FACTOR values in the user_tab_columns for my database table had bad
values. These values are calculated by the DBMS_STATS package. I manually set
these to a much lower figure and my problem went away.
I'm still trying to determine what my next steps are. One definite step is
to stop gathering stats for awhile!
Thanks
Tom
------------------------------------------------------------------------------
From: Koppelaars, Toon [mailto:T.Koppelaars@xxxxxxxxxxxxxxxxxxxx]
Sent: Tuesday, October 02, 2007 1:59 PM
To: Mercadante, Thomas F (LABOR); oracle-l@xxxxxxxxxxxxx
Subject: RE: Very Strange Query Access Plan
Can you provide us with a (couple of) example(s) of these queries?
-----Oorspronkelijk bericht-----
Van: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]Namens Mercadante, Thomas F (LABOR)
Verzonden: dinsdag 2 oktober 2007 17:12
Aan: oracle-l@xxxxxxxxxxxxx
Onderwerp: Very Strange Query Access Plan
All,
Running on 9.2.0.7 on Aix.
I have a database table with 18 million rows in it. There is an SSN column
in the table. Only 625,000 rows have an SSN populated. I also have an index
on this column. Statistics on both the table and index are gathered nightly
(using dbms_stats with the estimated and default sample options).
Yesterday, queries against this table used the index. Today, they do not.
There was no large data load overnight. We add about 10,000 records per day.
This behavior has happened before where the index gets ignored for a day and
then used the next day.
I really need to use an index here. Queries via ssn are executed all day
long and take 10 minutes to complete. CPU is pegged at 99%.
What am I missing? Maybe histograms? Haven't used them before, so any
suggestions would be helpful.
Thanks
Tom
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