Answer / thanjairajan@gmail.com

You Can use Bitmap Index. Because while no.of unique record is small than no.of total record, we can use bitmap index. i.e we can use bitmap index in low cardinality column.

For example a table contain name,address,sex column. in sex column may be two values. i.e male and female. so this distinct values small but no.of total records is high.

So in this situation we can use bitmap index.

Answer / mrityunjay singh

Fully indexing a large table with a traditional B-tree index can be prohibitively expensive in terms of disk space because the indexes can be several times larger than the data in the table. Bitmap indexes are typically only a fraction of the size of the indexed data in the table.

An index provides pointers to the rows in a table that contain a given key value. A regular index stores a list of rowids for each key corresponding to the rows with that key value. In a bitmap index, a bitmap for each key value replaces a list of rowids.

Each bit in the bitmap corresponds to a possible rowid, and if the bit is set, it means that the row with the corresponding rowid contains the key value. A mapping function converts the bit position to an actual rowid, so that the bitmap index provides the same functionality as a regular index. Bitmap indexes store the bitmaps in a compressed way. If the number of distinct key values is small, bitmap indexes compress better and the space saving benefit compared to a B-tree index becomes even better.

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