To enforce the uniqueness we only allow mapping of each unique/primary key value to one partition.

If we removed this limitation it would mean that for every insert/update we need to check in every partition to verify that it is unique. Also PK-only lookups would need to look into every partition.

What could be done is to enhance partitioning with Global Indexes, so that the unique/primary key is partitioned by those fields (or not partitioned at all) and the rest of the data are partitioned on a field not part of the unique/primary key. But this would mean that dropping a 'data' partition would need to do row-by-row deletes in the unique/primary key partitions/index. Also a PK-only lookup which accesses the whole row would need two lookups, the first on the PK index, and then on the 'data' index to retrieve the rest of the row.