Re: Histograms on VARCHAR2 columns ?

The metalink note is out of date - newer versions of Oracle use the first 31 characters to build histograms. Of course, if you have a column of URLs for Google search results which all start with:

http://groups.google.com/groups?hl=en&group=comp.databases.oracle.serv er

then you may have to rethink your design a bit.

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: 10 February 2003 03:15

>
>
>I have just begun trying Column histograms and have
>seen a better execution plan for a query
>on LOTRUN.RECTYPE when
> a) LOTRUN had about 5million records
> b) RECTYPE='A' is only 1570 records
> c) RECTYPE='H' is the other 5mn-1570 records
>We wanted the RECTYPE Index to be used because all
>of our queries are on RECTYPE='A'
>The RBO would do an Index Range Scan which was quite good
>However, the CBO under CHOOSE would do an Index Fast Full
>Scan for both values.
>
>Once I collected column statistics with
>ANALYZE TABLE LOTRUN COMPUTE STATISTICS FOR COLUMNS RECTYPE SIZE 10
{I know, I don't need 10 buckets},
>a query for RECTYPE='A' does an Index Range Scan with 10 consistent
gets while the query for RECTYPE='H' does an
>Index Fast Full Scan with 10060 consistent gets.
>
>Good !
>So far so good, I should say.
>
>I look at Note 72539.1 on MetaLink and I find this paragraph :
>
> STORING CHARACTER VALUES IN HISTOGRAMS
> --------------------------------------
>
> Character columns have some exceptional behaviour, in as much as we
store
> histogram data for the first 5 bytes of any string. Any predicates
that
> contain strings greater than 5 characters will not use histogram
information
> and the selectivity will be 1 / DISTINCT.
>
>
>
>Does this mean that a column with, say, 10 or 12 character
>values but with the same character-string in the first
>5 positions would not get meaningful histogram statistics ?
>
>
>Hemant K Chitale
>http://hkchital.tripod.com
>--
>Please see the official ORACLE-L FAQ: http://www.orafaq.net
>--
>Author: Hemant K Chitale
> INET: hkchital_at_singnet.com.sg
>
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-- 
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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