Unique Primary Indexes (UPIs) guarantee uniform distribution of table rows.
Nonunique Primary Indexes (NUPIs) can cause skewed data. While not a guarantor of uniform row distribution, the degree of uniqueness of the index will determine the degree of uniformity of the distribution. Because all rows with the same PI value are distributed to the same AMP, columns with a small number of distinct values that are repeated frequently do not make good PI candidates.
The most efficient access methods to get data in a table is through the PI. For this reason, choosing a PI should take the following design goal into consideration: choosing a PI that gives good distribution of data across the AMPs must be balanced against choosing a PI that reflects the most common usage pattern of the table.
If you do not explicitly specify a primary index when a table is created, Teradata Database uses the first column as the nonunique primary index by default.
User tables can also be created explicitly without a primary index. These NoPI tables are used for special purposes. NoPI tables are typically used as staging tables to allow faster data loading.