Hi all, probably a simple answers to these.

I some queries regarding the old phrase "if the optimiser determines
that a query would benefit from an index, it uses one".

These questions assume that no hints have been dropped in.

1. Does anyone have the very rough parameters/formulae that the
optimiser uses to determine when an index should be used?

2. Does anyone know how the optimiser determines where to find
information about what indexes are to be used? Would it be a simple matter
of making a query against the dictionary looking for indexes with column
structures matching the columns required?

3. Apart from the obvious reason that full-scan is more efficient,
are there any other reasons the optimiser should abandon an index in favour
of a full scan.

Thanks in adv.
Gj


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