This is a complex question that can't be easily answered in
an email, at least not by me. :)

Read the 'Optimizer' chapter in 'Designing and Tuning for Performance'
manual, it gives very good explanations of what drives the RBO
and CBO to use indexes, or not.

Jared


On Thursday 31 May 2001 07:37, Johnson, George wrote:
> 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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