Choosing the best indexing strategy


Oracle’s default index type - the B*Tree index is suitable for improving access for a wide range of queries. B*Tree indexes can optimize exact lookups and range quires and can sometimes be used to resolve queries in their own right and without reference to the underlying table.

The hash cluster allows you to store table data in a location that is derived from a mathematical manipulation of a key value. Hash clusters can improve access for exact key lookups but cannot enhance range queries and require careful sizing to prevent degradation of the hash cluster.

Bitmap indexes are useful to optimize quires in which multiple columns of low cardinality (few distinct values_ are queried in combination. Unlike B*-tree indexes, bitmap indexes can work of any combination of columns but increase locking contention.
DBA Tips Data Pump Reference

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