Indexes are usable (default) or unusable, visible (default) or invisible.

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These properties are defined as follows:

• UsabilityAn unusable index, which is ignored by the optimizer, is not maintained by DML operations. An unusable index can improve the performance of bulk loads. Instead of dropping an index and later re-creating it, you can make the index unusable and then rebuild it. Unusable indexes and index partitions do not consume space. When you make a usable index unusable, the database drops its index segment.
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• VisibilityAn invisible index is maintained by DML operations, but is not used by default by the optimizer. Making an index invisible is an alternative to making it unusable or dropping it. Invisible indexes are especially useful for testing the removal of an index before dropping it or using indexes temporarily without affecting the overall application.
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See also

“Overview of the Optimizer” to learn about how the optimizer select execution plans

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