Re: HISTOGRAMS , CBO and OLTP

Uniqueness indicates that there are less skewed values correct! Under what circumstances would a skewed column value for a unique column exist.

Another question, what about histograms on indexes itself? If histograms are generated on columns with skewed values, does it matter when the indexes are generated, before or after? Do indexes on these columns need to be rebuilt.

Thanks    

 Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk> wrote:

It's worth mentioning a frequently overlooked point that even if a column has a unique constraint (whether or not declared and/or supported by an index) then if it has a
skewed pattern of values the optimiser
can use a histogram to optimise a queries that target a range.

(NB The grammar suggests that the word
"not" was intended as the last word of the first line of (b) below).

Regards

Jonathan Lewis
http://www.jlcomp.demon.co.uk

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The Co-operative Oracle Users' FAQ
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-----Original Message-----
To: Multiple recipients of list ORACLE-L Date: 08 February 2003 10:44

>
>b) The number of distinct/duplicate values should
>really be relevant - its whether you will regularly
>need to probe a table using column values that are
>skewed in such a way as to have the optimizer make
>poor assumptions about their distribution.
>

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