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Home -> Community -> Mailing Lists -> Oracle-L -> Re: ** histograms
At 10:49 AM 11/19/2006, A Joshi wrote:
>Hi,
> About use of histograms : I think histograms are useful for
> indexes on columns that are not very selective.
Histograms have nothing to do with indexes. A histogram can be useful
to the CBO (the RBO does not use histogram information) if
(a) the column is used in a predicate
(b) the distribution of column values differs significantly from
uniform which the CBO assumes in the absence of a histogram
to "prove" the point that histograms have nothing to do with indexing I have an example where a histogram on a non-indexed column reduces cpu time and elapsed time of a query by by 50% and 40% respectively, i.e. the sql runs almost twice as fast with the histogram and consumes half as much cpu compared to without the histogram.
>However I came across note 1031826.6 on metalink. About maintenance
>and space cost. I think space cost is negligible and can be ignored.
>About maintenance : does it mean statistics need to be gather often?
>Or does it mean some other cost.
>
>Question : Is there any other overhead or any other negative impact
>of using histograms?
Yes
>
>Is it advisable to use histograms just for some tables and some
>specific columns or is it OK to just set database wide?
No.
At least in my opinion. I have seen an unnecessary histogram - on a column with perfectly uniform data distribution - destroy ( and I mean destroy, by a factor of ~ 9000 ) the performance of a sql statement. See also the recent post by Fuad Arshad and I quote: "it is generating histograms which doesnt work well with the application".
Regards
Wolfgang Breitling
Centrex Consulting Corporation
www.centrexcc.com
-- http://www.freelists.org/webpage/oracle-lReceived on Mon Nov 20 2006 - 08:51:22 CST
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