Adding to Steve's response: As Steve mentioned that you would need to enumerate all entities and then figure out unique PartitionKeys. However if you use query projection and only fetch just the PartitionKeys ($select=PartitionKey), your response size
will be much more smaller than fetching all attributes of your entities. Not sure about the performance when you fetch only few attributes though.
Marked As Answer byshrimpyFriday, April 27, 2012 8:05 AM