Oracle Solution for Error ora-14064

ORA-14064: Index with Unusable partition exists on unique/primary constraint key

What triggered the Error:

User attempted to add or enable a primary key/unique constraint on column(s) of a table on which there exists an index one or more partitions of which are marked Index Unusable.

What should we do to fix it:

Drop the existing index or rebuild unusable partitions it using ALTER INDEX REBUILD PARTITION