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Oracle 8 Tips  

by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Oracle Bitmapped Indexes

Prior to release 7.3 of Oracle, it was never recommended that the Remote DBA create an index on any fields that were not “selective” and had less than 50 unique values. Imagine, for example, how a traditional b-tree index would appear if a column such as REGION were indexed. With only four distinct values in the index, the SQL optimizer would rarely determine that an index scan would speed up a query; consequently, the index would never be accessed. Of course, the only alternative would be to invoke a costly full-table scan of the table. Today, we are able to use bitmapped indexes for low cardinality indexes. Cardinality is defined as the number of distinct key values expressed as a percentage of the number of rows in the table. Hence, a million row index with four distinct values has a low cardinality while a 100 row table with 80 distinct values has a high cardinality.

It is interesting to note that bitmapped indexes have been used in commercial databases since Model 204 was introduced in the late 1960s. However, their usefulness had been ignored until the data warehouse explosion of 1994 made it evident that a new approach to indexing was needed to resolve complex queries against very large tables.

Bitmapped indexes are a new feature of Oracle 7.3 that allow for very fast Boolean operations against low cardinality indexes. Complex AND and OR logic is performed entirely within the index--the base table need never be accessed. Without a bitmapped index, some decision support queries would be impossible to service without a full-table scan.

This is an excerpt from "High Performance Data Warehousing", copyright 1997. To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:



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