Re: CBO Predicate selectivity

Until 10g, the optimizer does not peek at values for this query - it uses the standard col >/< :bind algorithms which basically means the predicate has a selectivity of 0.25% (5% of 5%).

> - "where date between to_date(:b1) and to_date(b2)"

You say you can't change the query, so if you want to fake this query, you probably need to use dbms_stats.set_table_stats to tell Oracle that the table has a very small number of rows - but don't change the block count - so that a properly calculated selectivity is fooled into getting the right cardinality. Then capture the execution plan in a stored outline - and put the stats back to normal.

Regards

Jonathan Lewis
http://www.oracle.com/technology/community/oracle_ace/ace1.html#lewis

The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html

Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html

• Original Message ----- From: "FreeLists Mailing List Manager" <ecartis_at_freelists.org> To: "oracle-l digest users" <oracle-l_at_freelists.org> Sent: Thursday, July 13, 2006 7:05 AM

> Subject: CBO Predicate selectivity
> Date: Wed, 12 Jul 2006 08:44:18 -0000
> From: "Laimutis Nedzinskas" <Laimutis.Nedzinskas_at_landsbanki.is>
>
> Back to the basics.
>
> I need any information on how CBO calculates [predicate] selectivity
> which is more precise than in the Note:68992.1
>
> This note states that "c1 > :bind1 Default of 5%" which I
> doubt very much. At least it is not clear how distinct values/min-max
> values/datatype contribute to those 5% but the they do seem to
> contribute.
>
> As a whole, CBO uses some heuristics (see Note:212809.1) which I need to
> tweak by manipulating statistics for one simple reason:
>
> - I have a legacy query "where date between to_date(:b1) and
> to_date(b2)" => this query can not be modified.
>
> - I know what CBO does not know: an index on date column must be used
> because selectivity of "date between to_date(:b1) and to_date(b2)" is
> high.
>
>
> Thank you in advance,
> Laimis
> Fyrirvari/Disclaimer
> http://www.landsbanki.is/disclaimer
>
>

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