What are the types of PI (Primary Index) in Teradata?

The Teradata Primary index is not an index in the traditional sense, as it is not a lookup table. Instead, it is a mechanism that defines where each data row is physically located on the Teradata system. The primary index of a table may be defined as either a single column or as multiple columns. The values of the primary index columns within the table may be unique or non-unique. 

The Primary Index of a table should not be confused with the primary key of a table.The primary index is a part of the physical database model, and affects the storage and retrieval of data rows. The primary key is a part of the logical database model, and uniquely identifies each record in the table. Often, the primary key of a table is a good candidate for the primary index of a table, particularly for smaller “dimension” or “lookup” tables, but this is not always the case for other tables.

There are two types of Primary Index. Unique Primary Index ( UPI) and Non Unique Primary Index (NUPI). By default, NUPI is created when the table is created. Unique keyword has to be explicitly given when UPI has to be created.

UPI will slower the performance sometimes as for each and every row , uniqueness of the column value has to be checked and it is an additional overhead to the system but the distribution of data will be even. 

We should be careful while choosing a NUPI so that the distribution of data is almost even . UPI/NUPI decision should be taken based on the data and its usage.