Deferrable unique & primary keys must use nonunique indexes. - Oracle Schema

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• Nirja Shah   -Posted on 28 Sep 15
  - A user sees inconsistent constraints, including duplicates in unique indexes, when that user's transaction produces these inconsistencies.
  
  - You can place deferred unique and foreign key constraints on materialized views, allowing fast and complete refresh to complete successfully.
  
  - Deferrable unique constraints always use nonunique indexes.
  
  - When you remove a deferrable constraint, its index remains.
  
  - This is convenient because the storage information remains available after you disable a constraint.
  
  - Not-deferrable unique constraints and primary keys also use a nonunique index if the nonunique index is placed on the key columns before the constraint is enforced.

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