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 doesn't.
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.
Creation
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 CUSTOMERS table:
CREATE BITMAP INDEX cust_sales_bji
ON sales(customers.state)
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:
SELECT SUM(sales.dollar_amount)
FROM sales, customer
WHERE sales.cust_id = customer.cust_id
AND customer.state = 'California';
When dealing with large datasets, this reduction in processing can be substantial.
Restrictions
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 join index.
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 constraints.
If a dimension table has composite primary key, each column in the primary key must be part of the join.
