Re: re "CBO - A Configuration Roadmap" -- Histograms on Non-Indexed Columns

At 09:12 AM 12/28/2006, Hemant K Chitale wrote:
Can you explain the second sentence about Histograms on non-indexed columns as well ? How would they be useful ? I would understand Histograms on non-indexed columns as providing _me_ information about the data in those columns and which could allow me to make a judgement call as to whether I should index the columns. How does the 
optimizer use Histograms on non-indexed columns ?
Column statistics give the CBO information about the selectivity of predicates on that column. Predicate selectivity in turn gives rise to cardinality estimates. Differences in selectivity, e.g. by having a histogram, lead to differences in cardinality estimates which in turn can, and does, lead to different execution plans and peformance. For a very simple example see my paper on "Histograms - Myths and facts". 


Regards

Wolfgang Breitling
Centrex Consulting Corporation
www.centrexcc.com 


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