Teradata takes each table and spreads the table rows across the AMPs. When the table needs to be read, each AMP has to read only their portion of the table. If the AMPs start reading at the same time and there are an equal amount of rows on each AMP, then parallel processing works brilliantly. Alone an AMP can do so little, but the AMPs working together can accomplish the incredible. This brilliant feat begins with the Primary Index. Each table in Teradata is required to have a Primary Index. The biggest key to a great Teradata Database Design begins with choosing the correct Primary Index. The Primary Index will determine on which AMP a row will reside. Because this concept is extremely important, let me state again that the Primary Index is the only thing that will determine on which AMP a row will reside.
Many people new to Teradata assume that the most important concept concerning the Primary Index is data distribution. INCORRECT! The Primary Index does determine data distribution, but even more importantly, the Primary Index provides the fastest physical path to retrieving data. The Primary Index also plays an incredibly important role in how joins are performed. Remember these three important concepts of the Primary Index and you are well on your way to a great Physical Database Design.
The Primary Index plays 3 roles: Data Distribution Fastest Way to Retrieve Data Incredibly important for Joins
Tips: Because the Primary Index is so important it should NOT be frequently changed. It should also be a column that is frequently used in row selection. This means that the SQL should utilize this column often in the WHERE clause. Although you might be tempted to pick a column with poor distribution you should not. This can cause errors stating Database FULL Conditions.