Oracle 10 optimizer from RULE to COST: why?

Question

Oracle decided to dismiss the rule-based optimizer from version 10g, leaving the cost-based one as the only choice.

I think that a rule-based optimizer has the unv

Answer 1

(I am not a DBA.)

My understanding is that Oracle has been moving away from the RBO for a long time in favor of CBO. It seems useful to me to stop supporting a feature that is no longer in active development (given a long enough depreciation period) so that everyone is using the most effective features.

It's interesting that you called predictability an "unvaluable" effect of using the rule-based optimizer. It seems like when the data changes to make an execution plan sub-optimal it would be best to switch to a new one. Only in the case you alluded to where the optimizer flip-flops between two execution plan would there be a problem with picking the best plan for the the data you are actually querying. I'm not sure what advantage predictability is in more normal situation.

Ending support of the out-dated optimizer ought to free up support for the newer optimizer.