Fwd: Histograms on VARCHAR2 columns ?

Resending and hoping for some responses, even some real-world stories ... ?...

>Date: Sun, 09 Feb 2003 18:38:35 -0800
>To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
>X-Sender: Hemant K Chitale <hkchital_at_singnet.com.sg>
>Subject: Histograms on VARCHAR2 columns ?
>
>
>
>
>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
My web site page is : http://hkchital.tripod.com

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Author: Hemant K Chitale
  INET: hkchital_at_singnet.com.sg

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