15.00 - General Considerations - Teradata Database

Teradata Database Design

prodname
Teradata Database
vrm_release
15.00
category
User Guide
featnum
B035-1094-015K

General Considerations

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 definition.
  • 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.