Re: HISTOGRAMS , CBO and OLTP

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
http://www.jlcomp.demon.co.uk/faq/ind_faq.html

-----Original Message-----
To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com> 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.
>

-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: Jonathan Lewis
  INET: jonathan_at_jlcomp.demon.co.uk

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