A unique primary index (UPI) requires each row in a table to have a unique value for the combined index columns. A UPI often corresponds to the primary key constraint or unique constraint. Because a UPI has a unique value for each row, table rows are typically evenly distributed.
UPIs can cause a skewed data distribution if the number of rows in the table is small relative to the number of AMPs in the map used by the table. You can create a map that contains fewer AMPs and assign that map to the table.