Re: Index skip scan

The answer is similar to the one for the question "What is the % of rows where the optimizer switched from an index access to a full scan, or vice versa". There is not fixed threshold. The optimizer with cost the various possible access paths and choose the one the comes out with the lowest cost. BTW, skip scan is not limited to just skipping one leading column. You can get an index skip-scan with predicates missing more than one leading column of the index.

Quoting "Hameed, Amir" <Amir.Hameed_at_xerox.com>:

> Folks,
> Does anyone know that in order for the optimizer to choose "Index skip
> scan" on a concatenated index where the first column is not very
> selective but the second one is, approximately what type of selectivity
> on the leading column the Optimizer look for before deciding to take
> this route?

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regards

Wolfgang Breitling
Oracle 7,8,8i,9i OCP DBA
Centrex Consulting Corporation
www.centrexcc.com

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