Currently

hibernate.schema_update.unique_constraint_strategyorg.hibernate.tool.hbm2ddl.UniqueConstraint

can assume 3 values

• DROP_RECREATE_QUIETLY, the default value
• RECREATE_QUIETLY
• SKIP

with the first 2 values any exception during the constraint drop or creation is silently ignored.

The introduction of 2 other values

• DROP_RECREATE
• RECREATE

wants to add the possibility to not ignore exceptions during the constraint drop or creation.