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More Than One Kind of Index

All database vendors support B-Tree indexes, and a few support bitmap indexes.

A B-Tree index is an ordered set of entries. Each entry contains a search-key value and a pointer to a row in the table that contains the value. The server can attack the ordered structure of B-Trees predictably and efficiently, and a B-Tree index is smaller than the underlying table. B-Tree indexes improve performance most when a column contains mostly distinct values (index cardinality is low). They are also used to enforce uniqueness.

A bitmap index is also a set of entries. Each entry contains a search-key value and a bitmap (or pointer to a bitmap). Each bit corresponds with a row in the table, and the presence of a bit indicates the row contains the search-key. Bitmap indexes are often useful in many situations where B-Tree indexes are not. Namely, for columns which have a large number of duplicate values (index cardinality is high). For example, a Size column that contains only five distinct values (tiny, small, medium, large, grand) is a good candidate. Oracle Corporation supports bitmap indexes.


 Siebel Analytics Performance Tuning Guide 
 Published: 18 April 2003