17.10 - Index Hash Mapping - Advanced SQL Engine - Teradata Database

Teradata Vantage™ - SQL Fundamentals

Product
Advanced SQL Engine
Teradata Database
Release Number
17.10
Release Date
July 2021
Content Type
Programming Reference
Publication ID
B035-1141-171K
Language
English (United States)

Rows are distributed across the AMPS using a hashing algorithm that computes a row hash value based on the primary index. The row hash is a 32-bit value. Depending on the system setting for the hash bucket size, either the higher-order 16 bits or the higher-order 20 bits of a hash value determine an associated hash bucket.

Normally, the hash bucket size is 20 bits for new systems. If you are upgrading from an older release, the hash bucket size may be 16 bits. If a 20-bit hash bucket size is more appropriate for the size of a system, you can use the DBS Control Utility to change the system setting for the hash bucket size.

IF the hash bucket size is … THEN the number of hash buckets is …
16 bits 65536
20 bits 1048576

The hash buckets are distributed as evenly as possible among the AMPs on a system.

Vantage maintains a hash map—an index of which hash buckets live on which AMPs—that it uses to determine whether rows belong to an AMP based on their row hash values. Row assignment is performed in a manner that ensures as equal a distribution as possible among all the AMPs on a system.

Related Information

For details about hash bucket size, see the information about the DBS Control Utility in Teradata Vantage™ - Database Utilities, B035-1102.