A primary index (PI) or primary AMP index (PA) facilitates access to data and determines how table rows are distributed among the AMPs of the system. A properly chosen PI or PA helps make sure that rows are distributed evenly, so each AMP manages a similar amount of data.
A PI or PA is defined to use from one to 64 columns of a table. For each table row, the values of these index columns are combined and hashed. The resulting numeric hash value determines which AMP stores and manages the data in that row. For a PI, the hash value is also used in ordering rows on an AMP. A PA is only allowed for a column-partitioned table.