Why does an upgrade to NuoDB 2.5.6 requires the rebuild of unique and primary key indexes?
Due to the change DB-16260 all unique and primary key indexes must be rebuilt when upgrading from versions below 2.5.6 to 2.5.6 and above.
Unlike in pre-2.5.6 versions uniqueness is now consistently enforced and unique indexes will fail to rebuild if they contain duplicate values.
When upgrading it is necessary to identify and fix any duplicate values in columns that have unique indexes on them. How duplicate values should ultimately be resolved--altered, merged, or deleted--depends on what the data represents in the application.