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Bitmap indexes are most suitable for columns having very few unique values (very low cardinality). This is best for multiple combination of low to medium cardinality columns.
B-tree indexes are appropriate for medium to high cardinality columns having many distinct values.
Bitmap indexes are more advantageous than btree when the table has million of rows and the key column has low cardinality. bitmap indexes provide better performance than b tree indexes when queries often use a combination of multiple where conditions involving the OR operator, the table is read-only, or when there is low update activity on the key columns.