The following factors apply equally to Teradata Database UPIs and NUPIs.
Each table has 0 or 1, and no more than 1, primary index.
The primary index for a table need not be isomorphic with the primary key for the
table. This is obviously true for NUPIs, because primary key values are unique by
A primary index can contain nulls, while a primary key cannot.
Primary index nulls are not a good thing. If you anticipate that the primary index
for a table might frequently be null, you should consider using a different column.
All single‑value primary index accesses are confined to a single AMP. In most cases,
a single‑value primary index access requires only 1 I/O.
If the cylinder index is not in cache, a primary index access requires 2 I/Os.