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Bitmap Join Index

From: <aravind.kanda_at_gmail.com>
Date: 9 Feb 2007 06:17:56 -0800
Message-ID: <1171030676.189507.273130@a75g2000cwd.googlegroups.com>


I am working on creating a Bitmap Join Index and it does not seem to be working. Before I post the sql & explain plan here, I have few doubts that I'd like to get out of the way. These are based on some of my observations and so they could be wrong.

Bitmap Join Index in turn creates Bitmap indexes on the underlying tables. Since bitmap indexes are mostly meant for low cardinality columns, a bitmap join index which uses a join condition using a high cardinality column may not achieve desire results. For example, if I create a BMJ on Customers & States, it is likely to work since the states cannot be more 50. Whereas if I create BMJ on parts & suppliers where the unique identifiers can exceed potentially thousands of records, a BMJ does not make sense. Is that right?

When a query includes a where clause on a column other than those mentioned in the BMJ, then the BMJ is not invoked by the Optimizer. Is that correct? For example, if I create BMJ on Customer_summary & Customer, If I run a query to filter on a particular customer, lets say

select * from customer_summary, customer where customer_id = customer_id and customer_nm = 'Mr.AAA'

This is not going to work since there are additional filters and the optimizer may skip the BMJ. Is this correct?

Pl. help clear my misconceptions, if any. Received on Fri Feb 09 2007 - 08:17:56 CST

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