Hi,
There is quite the scenario that the Oracle Cost-Based Optimizer (CBO) uses when deciding when to use indexes to process your SQL. I go into this in great detail in my SQL Performance Tuning cours...
A couple of days ago the post man knocked on my door with a package. I hadn't ordered anything, so it was a puzzling what it might be.
When I opened the package I found 3 copies of a book in Chinese....