migration from RBO to CBO

Hi List,

I'm going to change my job and I'll have to inherite one DB which doesn't seem to be very good
designated. I haven't known much about the application yet and I can't post many details to you but
I would appreciate your general advices.

Despite of the optimizer_mode set to RULE at instance level, they use many new features
(partitioning, bitmap indexes, function-based indexes, etc). It seems the system is
overloaded with
indexes and optimizer hints. They collect statistics regularly. Molotov's cocktail! So I expect some
hybrid behaviour of the optimizer. The sql statements don't seem to be especially optimized for RBO
and give real trouble for system (my first feeling).

Before we could start any sql tuning we have to decide which road to choose. My current understanding is that we need to fix existing execution plans with 'Plan Stability' option and set
optimizer_mode = all_rows. Then ask the developers to use CBO in new development and gradually test
and rewrite existing code. What would be your advice? Any experience?

Also I would like to know if anybody use the 'plan stability' feature. Is it stable itself ? :) The
system is 8.1.7.0.1 on Red Hat

Thanks,
Ed

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Author: Shevtsov, Eduard
  INET: EShevtsov_at_flagship.ru

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