Storage Indexes are an extremely powerful feature of Oracle Exadata System Software. Storage Indexes enable the storage servers to avoid I/O operations on disk and flash regions that cannot contain data related to specific SQL queries. Managed automatically, Storage Indexes are memory structures that track minimum and maximum column values for the storage regions of each table on that storage server.
When SQL is executed by the Database, Exadata System Software interogates the Storage Index to determine if the contents of a region can satisfy the WHERE clause by comparing the column min/max values tracked in the Storage Index with the query predicates. If the predicate value falls within a regions min/max range, the region will be accessed and processed. However, if the predicate value falls outside the region min/max range, IO will be avoided. The resulting IO Pruning enabled by Storage Indexes can provide a significant improvement on performance on many workloads and enable more workloads to run on the same Exadata Database Machine.
Beginning in Exadata System Software 21.2.0, Storage Indexes can be persisted to the local SSD to eliminate the need to rebuild the Storage Index on Storage Server restart ensuring consistent query performance across server restarts.