Unique indexes are managed using the host index files to maintain values across all partitions for unique non-partition keys. The values in the host index use their partition number (plus 1) as their associated values. The partition index contains the key value with the associated record position within the data partition. This permits the partitioned files to be treated as a self-contained set of ISAM files, and still enforces unique keys across all partitions. Of course, there is a performance penalty associated with indexing the non-partition unique keys twice.
A duplicate key flag value, 2, permits unique keys within a partition, but no check is made for global uniqueness across all partitions.