bitmap index

[akhil]

Hi,
Can anyone tell me what is bitmap index. I will appreciate if an example is provided with the reply.

thankx in advance.

[shaju]

The advantage of bitmap index are greatest for low cardinality columns i.e, if the values in a column are repated more than a hundred times, the column is a candidate for a bitmap index

syntax:
CREATE BITMAP INDEX ON ();

EXAML\PLE:

CREATE BITMAP INDEX V1_INDX ON ORDERS(ORDERNO);

[sonuji_in]

hi akhil

The principle of a bitmap index is the use of a matrix, which has columns corresponding to all data values in the column. Thus, if the column contains only three distinct values, the bitmap index can be visualized as containing a column for the ROWID and three columns, one for each distinct value. Figure 4-3 displays a pictorial representation of a bitmap index containing three distinct values. The physical representation of the bitmap index is not far from the picture. Since each distinct value adds to the size of the index, bitmap indexes work best when there are few distinct values allowed for a column. Thus, the bitmap index improves performance in situations where B-tree indexes are not useful, and vice-versa.

Bit set in this column indicates row has this value set for the column

regards
Amit