Bitmap index is used for only more duplicate values in the column. Do not user on Unique column. because it will take more space and also more physical read for retrieve information than btree index.
Btree indexes used for columns which have unique values.
In summary, bitmap indexes are best suited for DSS regardless of cardinality for these reasons:
- With bitmap indexes, the optimizer can efficiently answer queries that include AND, OR, or XOR. (Oracle supports dynamic B-tree-to-bitmap conversion, but it can be inefficient.)
- With bitmaps, the optimizer can answer queries when searching or counting for nulls. Null values are also indexed in bitmap indexes (unlike B-tree indexes).
- Most important, bitmap indexes in DSS systems support ad hoc queries, whereas B-tree indexes do not. More specifically, if you have a table with 50 columns and users frequently query on 10 of themeither the combination of all 10 columns or sometimes a single columncreating a B-tree index will be very difficult. If you create 10 bitmap indexes on all these columns, all the queries can be answered by these indexes, whether they are queries on all 10 columns, on 4 or 6 columns out of the 10, or on a single column. The AND_EQUAL hint provides this functionality for B-tree indexes, but no more than five indexes can be used by a query. This limit is not imposed with bitmap indexes.
In contrast, B-tree indexes are well suited for OLTP applications in which users' queries are relatively routine (and well tuned before deployment in production), as opposed to ad hoc queries, which are much less frequent and executed during nonpeak business hours. Because data is frequently updated in and deleted from OLTP applications, bitmap indexes can cause a serious locking problem in these situations.
The data here is fairly clear. Both indexes have a similar purpose: to return results as fast as possible. But your choice of which one to use should depend purely on the type of application, not on the level of cardinality.
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