Performance problem in OLAP DB by using optimizer_index_cost_adj=1

Question

Hi,
I have Oracle 11gR2 as my DB and Siebel 8.1 Application
I would like to ask everyone about the significance of below two parameters:
1> optimizer_index_cost_adj=1
2> optimizer_index_caching=0
I'm facing performance issue while extracting data from my OLTP DB to OLAP DB. Below I have described it much briefly for your persual
I had noted that the explain plans to extract the data from OLTP are very bad to extract lots of OLTP data.
For example, the explain plan for one of our transaction called "extract person data" is mostly doing nested loops instead of hash joins. I checked some OLTP parameter and I noted one important Db parameter “optimizer_index_cost_adj “ has the current value 1.This leads to the nested loops as the DB is not acting cost based Since, Oracle recommends this paramter value to 1 for better performance of Siebel application.
Whereas, If I change the above paramters to below values
optimizer_index_cost_adj=25;
optimizer_index_caching=75;
and I reused this parameter as to create explain plans for the specific long running queries and they changed as expected.
Now, my question is if Oracle recommends 1> optimizer_index_cost_adj=1 and 2> optimizer_index_caching=0 then why the cost for many queries increase enormously which in turn leads to bad performance during extraction of dat from OLTP system to OLAP system?
Request you to please comment and suggest me how to proceed
Thanks,
Anil

Answer 1

Going from 10.2.x to 11.2.0.3 I have often noticed two points:
First, you must re-gather system statistics. If you run
select * from sys.aux_stats$;
what do you get? Have they ever been gathered? I suspect the 11.x makes more use of them than earlier releases.
Secondly, the default for serialdirect_read is now auto, in 10g it was (I think) false. This can have a big effect on pushing the optimizer away from indexes.