CREATE BITMAP INDEX

Specify BITMAP to indicate that index is to be created with a bitmap for each distinct
key, rather than indexing each row separately. Bitmap indexes store the rowids
associated with a key value as a bitmap. Each bit in the bitmap corresponds to a
possible rowid. If the bit is set, then it means that the row with the corresponding
rowid contains the key value. The internal representation of bitmaps is best suited for
applications with low levels of concurrent transactions, such as data warehousing.

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Note: Oracle does not index table rows in which all key columns
are null except in the case of bitmap indexes. Therefore, if you want
an index on all rows of a table, then you must either specify NOT
NULL constraints for the index key columns or create a bitmap
index.
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