Re: re "CBO - A Configuration Roadmap" -- Histograms on Non-Indexed Columns
- From: Wolfgang Breitling <breitliw@xxxxxxxxxxxxx>
- To: hkchital@xxxxxxxxxxxxxx
- Date: Thu, 28 Dec 2006 09:44:30 -0700
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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