Description: Oracle introduced a compression option for indexes in Oracle 8.1. You can create an index as compressed, or rebuild it to compress it.
Index compression is implemented at the index block level. Oracle stores each distinct combination of compressed column values found within a specific index leaf block in a "Prefix" table within the leaf block and assigns each combination a unique prefix number.
The benefits of compression come from the fact that a properly compressed index uses a smaller number of leaf blocks - which tends to mean that less I/O is involved when the index is used, there is a reduced amount of buffer cache flushing, and the optimizer is likely to calculate a lower cost for using that index for range scans.
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