Originally posted by learning_bee
I have two instances exactly the same on structure with slightly difference on data.
It's not just differences in the data that matter. Other issues that Oracle considers are ...

* The order of the data (as judged by the clustering factor on the index)
* What the statistics say about the index and the table, as opposed to what the actual data is.
* Values of initialization parameters.
* System statistics, if you have gathered them.

A good place to start learning would be to read the Oracle Performance Guide from http://tahiti.oracle.com, and the Concepts Guide.