Performance Considerations for Identity Columns
Identity columns have minimal cost with respect to system performance. However, the initial load of an identity column table might create a performance decrement because every AMP that has rows for an identity column table must reserve a range of numbers at about the same time.
When the table to be updated has a primary index, there is a performance degradation for INSERT and UPDATE operations if the primary index is an identity column. When an identity column is defined on a table column other than the primary index, the performance cost is negligible.
If you write applications that access tables with identity columns, coding application SQL requests to return values from the identity column improves the performance of open access products.