Re: bitmap index not used

Which version of the database? Do you have any special setup? Any optimizer parameters on either session or system level?
On 10/29/2003 03:09:39 PM, Josh Collier wrote:
> Hi,
>
> I have a puzzle.
>
> A bitmap index on a varchar2(25) column. table has 7131413 rows, of which
> 7125290 are null for the column in question, the rest of the rows are unique
> values. There is a histogram on this column, it has 2 buckets.
>
> A select statement using this column in the where clause generates two
> plans, one which uses the index and one which doesn't. The deciding factor
> is the length of the string in the filter.
>
> select * from table where microchip_number = 'avid1'
> this produces a plan which uses the bitmap
>
> select * from table where microchip_number = 'avid12' (and any additional
> characters in the string )
> this produces a plan which uses a full table scan.
>
> I am stumped about how to figure out why the optimizer makes a choice based
> upon the length of the string in the predicate.
>
> Much obliged,
>
> Josh
>
> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Josh Collier
> INET: Josh.Collier_at_Banfield.net
>
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Mladen Gogala
Oracle DBA

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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: Mladen Gogala
  INET: mladen_at_wangtrading.com

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