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RE: bitmap index not used

From: Khedr, Waleed <Waleed.Khedr_at_FMR.COM>
Date: Wed, 29 Oct 2003 13:49:29 -0800
Message-ID: <F001.005D4F7A.20031029134929@fatcity.com>


Probably the histogram is deciding that. Remove the histogram and see if there is any changes.

Waleed

-----Original Message-----
Sent: Wednesday, October 29, 2003 3:10 PM To: Multiple recipients of list ORACLE-L

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

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Author: Josh Collier
  INET: Josh.Collier_at_Banfield.net

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Author: Khedr, Waleed
  INET: Waleed.Khedr_at_FMR.COM
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