This task is mostly a re-engineering task.
Currently when calculating the cost of a ref table access method the function
best_access_path makes an assumption that any index look-up is performed into a
B-tree and the function uses engine independent code for this calculations.
The cost calculations also use some constants that should be done tunable
through system variables since these constants may be different for different
platform/environments.

This task is implemented in preparation for the development that will support
more realistic cost model for query execution plans.