diable index和unusable index

ENABLE Clause

ENABLE applies only to a function-based index that has been disabled because a user-defined function used by the index was dropped or replaced. This clause enables such an index if these conditions are true:

*The function is currently valid
*The signature of the current function matches the signature of the function when   the index was created
*The function is currently marked as DETERMINISTIC

Restriction on Enabling Function-based Indexes

You cannot specify any other clauses of ALTER INDEX in the same statement with ENABLE.

DISABLE Clause

DISABLE applies only to a function-based index. This clause lets you disable the use of a function-based index. You might want to do so, for example, while working on the body of the function. Afterward you can either rebuild the index or specify another ALTER INDEX statement with the ENABLE keyword.

alter index xx unusable;

UNUSABLE Clause Specify UNUSABLE to mark the index or index partition(s) or index subpartition(s) UNUSABLE. An unusable index must be rebuilt, or dropped and re-created, before it can be used. While one partition is marked UNUSABLE, the other partitions of the index are still valid. You can execute statements that require the index if the statements do not access the unusable partition. You can also split or rename the unusable partition before rebuilding it.