17.10 - UNIQUE Versus Nonunique Secondary Indexes - Advanced SQL Engine - Teradata Database

Teradata Vantage™ - SQL Data Definition Language Detailed Topics

Product
Advanced SQL Engine
Teradata Database
Release Number
17.10
Release Date
July 2021
Content Type
Programming Reference
Publication ID
B035-1184-171K
Language
English (United States)

The UNIQUE option creates a USI which, perhaps redundantly, prevents the occurrence of duplicate index values in the indexed base table. Only one row can have a value set in the column set that corresponds to an entry in the subtable of a unique secondary index. The subtable of a unique secondary index is a hashed table, which is very efficient for retrieving a single row.

When the values of nonunique secondary indexes are used to apply complex conditional expressions to very large tables, efficient retrieval is achieved by bit mapping the subtable row IDs.