Bitmap Index for Oracal 10g

[Deeptimoy]

hi i am bit interested to know about Bitmap Index as highly compressed index type that tends to be used primarily for data warehouses.i am not completed my oracal10g course,its currently going on but i come across this in my work so please help me to understand it.thank you.

[MindSpace]

Oracle's index types are Bitmap indexes and B-Tree indexes. B-Tree indexes are the regular type that OLTP systems make much use of, and bitmap indexes are a highly compressed index type that tends to be used primarily for data warehouses.

Characteristic of Bitmap Indexes

* For columns with very few unique values (low cardinality) Columns that have low cardinality are good candidates (if the cardinality of a column is <= 0.1 % that the column is ideal candidate, consider also 0.2% – 1%)

* Tables that have no or little insert/update are good candidates (static data in warehouse)

* Stream of bits: each bit relates to a column value in a single row of table

Advantage
The advantages of them are that they have a highly compressed structure, making them fast
Compressed indexes represent a trade-off between CPU usage and disk space usage.
can be used very successfully for indexing columns with many thousands of different values.

Disadvantage
The reason for confining bitmap indexes to data warehouses is that the overhead on maintaining them is enormous.
modification requires a great deal more work on behalf of the system than a modification to a b-tree index.

[opaper]

bitmap indexes are best suited for DSS.With bitmap indexes, the optimizer can efficiently answer queries that include AND, OR, or XOR.Most important, bitmap indexes in DSS systems support ad hoc queries.check here
http://www.oracle.com/technology/pub...a_indexes.html