A Unique Primary Index means that the values for the selected column must be unique. The data will always be spread evenly if the Primary Index is unique. If you try and insert a row with a Primary Index value that is already in the table, the row will be rejected. Even though a Unique Primary Index will always spread the table rows evenly amongst the AMPs, please dont assume this is always the best thing to do.
A Unique Primary Index (UPI) will always spread the rows of the table Evenly amongst the AMPs. UPI access is always a one-AMP operation. It also requires no duplicate row checking.
Non-Unique Primary Index:
A Non-Unique Primary Index (NUPI) means that the values for the selected column can be non-unique. You can have many rows with the same value in the Primary Index. A Non-Unique Primary Index will almost never spread the table rows evenly. Please dont assume this is always a bad thing.