Oracle-L: RE: DBMS_STATS and CBO

From: <Govind.Arumugam_at_alltel.com>
Date: Fri, 08 Aug 2003 20:14:23 -0800
Message-ID: <F001.005C9B7A.20030808201423@fatcity.com>

Prasad,

We ran into the same problem when we did FOR ALL INDEXED COLUMNS using DBMS_STATS. Then we changed it to run against FOR ALL COLUMNS SIZE 1. Then CBO started to use the indexes.

execute dbms_stats.gather_table_stats('owner','table',NULL,30,TRUE, 'FOR ALL COLUMNS SIZE 1',20,'DEFAULT',TRUE); Here 30 is the sample percent (ie 30%) and 20 is the the parallel degree.

Hope this helps.

Govind

-----Original Message-----
[mailto:Prasada.Gunda_at_hartfordlife.com]
Sent: Friday, August 08, 2003 9:24 PM
To: Multiple recipients of list ORACLE-L

We have a table (monthly fact table) which has 24 partitions and partitioned by month. There is data in only 3 partitions. All the indexes are locally partitioned.

In dev db, I analyzed the table and indexes with analyze table..compute statistics. When I query the no. of rows group by month, it returns the query in couple of seconds and does the index scan(bitmap) on month column.

I created this table in production db and this time I used the dbms_stats to create the statistics (compute both on table and indexes). When I run this query, it does full table scan. The only way I could make it to use index scan by specifying hints.

All the parameters(init.ora) are exactly the same on both databases and it is 8.1.7.4. In prod db, I tried various combinations of optimizer_index_caching, optimizer_index_cost_adj to favor the index scan. There was no use.

Then, I did analyze table .. compute statistics on prod table but it was still doing the FTS. I did not delete the stats created by dbms_stats before using analyze table stmnt.

Finally, I deleted the stats generated by dbms_stats/analyze table before generating stats again with Analyze table <table> compute statistics. It is doing the index scan now.

I have no clue 1. why it does not use index when I generated the stats w/dbms_stats. 2. why I had to explicitly delete the stats before generating the stats again to make use of the index.

Thanks for your help in advance.

Thanks.

Best Regards,
Prasad
860 843 8377

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Author: <Govind.Arumugam_at_alltel.com
INET: Govind.Arumugam_at_alltel.com

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