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When Not to Use
Bitmaps have poor locking characteristics
A DML lock in a bitmap index will lock many rows
Columns with good selectivity such as customer number or social security number are better served by B-Tree indexes
Notes:
A DML lock in a bitmap index will lock many rows unlike a B-Tree index where a single row is locked.
A DML lock in a bitmap index will lock many rows:
This is because a bitmap entry is typically made up of a range of ROWIDS, not a single rowid
when a bitmap index entry is locked, the entire range of ROWIDS is locked
bitmap index locking may heavily impact OLTP systems
AVOID IF HEAVY DML IS EXPECTED