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Bitmap Join Indexes
Bitmap index for the join of two or more tables
Intended for data warehouse
- Fact to Dimension table join
- DIMENSION.PK = FACT.FK
Pre-computes the join operation
Reduce or sometimes eliminate join during query
Bitmap structure is space efficient
Comes with a price
Notes:
In addition to a bitmap index on a single table, you can create a bitmap join index, which is a bitmap index for the join of two or more tables. A bitmap join index is a space efficient way of reducing the volume of data that must be joined by performing the join operation in advance, I.e. at index creation time.
Bitmap join indexes are intended for use in a data warehouse type of environment where you have dimension tables and fact tables. In such an environment, the bitmap join index would store the value of a column from a dimension table along with the rowid(s) of the like value(s) from the other table(s) that the index is defined on. However, they can work in any environment where a PK to FK relationship exists.
Bitmap join indexes efficient in terms of space required to store the object. You may be thinking that these indexes are comparable to a materialized view based on a join. The difference is that the materialized view actually creates a table – which typically requires lots of disk space. The bitmap join index requires only a fraction of the space.