In Oracle8i performance improvements were made using materialized views to store the resulting rows
of queries. The benefits of this mechanism are still relevant, but a certain subset of the queries
used in a data warehouse may benefit from the use of Bitmap Join Indexes.
How It Works
In a Bitmap Index, each distinct value for the specified column is associated with a bitmap where
each bit represents a row in the table. A '1' means that row contains that value, a '0' means it
Bitmap Join Indexes extend this concept such that the index contains the data to support the join
query, allowing the query to retrieve the data from the index rather than referencing the join
tables. Since the information is compressed into a bitmap, the size of the resulting structure is
significantly smaller than the corresponding materialized view.
The index is created with reference to the columns in the joined tables that will be used to support
the query. In the following example an index is created where the SALES table is joined to the
CREATE BITMAP INDEX cust_sales_bji
FROM sales, customers
WHERE sales.cust_id = customers.cust_id;
Since the CUSTOMERS.STATE column is referenced in the ON clause of the index, queries on the SALES
table that join to the CUSTOMERS table to retrieve the STATE column can do so without referencing
the CUSTOMERS table. Instead the data is read from the bitmap join index:
WHERE sales.cust_id = customer.cust_id
AND customer.state = 'California';
When dealing with large datasets, this reduction in processing can be substantial.
Bitmap Join Indexes have the following restrictions:
* Parallel DML is currently only supported on the fact table. Parallel DML on one of the
participating dimension tables will mark the index as unusable.
* Only one table can be updated concurrently by different transactions when using the bitmap
* No table can appear twice in the join.
* You cannot create a bitmap join index on an index-organized table or a temporary table.
* The columns in the index must all be columns of the dimension tables.
* The dimension table join columns must be either primary key columns or have unique
* If a dimension table has composite primary key, each column in the primary key must be part of
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