FreeLists / oracle-l / RE: Very Strange Query Access Plan

["Mercadante, Thomas F (LABOR)" <Thomas.Mercadante@xxxxxxxxxxxxxxxxx>]

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