Asad

• Jul 30th, 2007

 

Index are reference to data in tables's columns on what they are based. Index work exactly in way  like we have book's index describing its chapters and what page # chapter can be found.

They only contain reference to the data. There are two types on index.

B-tree index and bit map indexes.

Where data cardinality (uniqueness) is low, bitmap is good candidate for the data otherwise b-tree indexes would help. They are used to access data through reference (row-id) which is fastest method to access data on disk.

Index also affect optimizer decision as how to access data. choice of index (bitmap or b-tree) can be also dependant on application (either DSS or OLTP).