Oracle Siebel Tuning Tips

Oracle Database Tips by Donald Burleson

Question: I have a Siebel database coming from a Rule based optimizer release in 8.1.7.4 to 10gR2. I've followed the docs presented by Siebel, which involves a lot of changes to the optimizer to force it to behave more like the RBO (They even set the optimizer mode in the session to FIRST_ROWS_10). With the supplied init.ora changes and dbms_stats w/histograms performance was pretty lousy, but by changing only the optimizer_features_enable back to 8.1.7 performance was much better.

Answer: It's always difficult tuning a vendor application when you do not have access to the source code SQL, but I have some tips in my book "Oracle Tuning: The Definitive Reference", on using stored outlines and SQL profiles.

In this note we see that some Oracle Siebel users are deliberately going un-supported in order to move their Siebel database from the obsolete rule-based SQL optimizer (RBO) to cost-based SQL optimization. They report that his Oracle Siebel Database ran far faster when he changed his optimizer_mode and analyzed statistics:

"While using the RBO our OLTP type queries were measured in minutes and after analyzing and using the CBO, execution time dropped to several seconds; a 30-60 fold increase in performance."

It also reports that Oracle Siebel databases will benefit from using function-based indexes (FBI):

"Once we were on the CBO we felt our hands were untied and we started to add function based indexes for case insensitive searches, partitioning etc so our execution time for most queries became sub-second."

Of course, customizing an Oracle Siebel database by adding indexes and using cost-based SQL optimization may make your Siebel Database unsupported, but this person felt it was worth it:

"Siebel was not happy to say the least and they said they wouldn't support us, my company decided such a performance gain was worth canceling support so we got the performance increase and saved ourselves a lot of cash at the same time."

Also, Siebel recommends setting the optimizer_index_cost_adj parameter to a super-low value of one, strongly recommended by Siebel:

"Optimizer_index_cost_adj=1 is strongly recommended by Siebel no exception. We have changed it to 100 and it degraded performance."

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