16.10 - Finding Uneven Distribution Using Hash Functions - Teradata Database

Teradata Database Administration

Product
Teradata Database
Release Number
16.10
Release Date
April 2018
Content Type
Administration
Publication ID
B035-1093-161K
Language
English (United States)

Use the following functions to identify uneven hash distribution of data.

Function Definition
HASHAMP AMP that owns the hash bucket
HASHBACKAMP Fallback AMP that owns the hash bucket
HASHBUCKET Grouping for the specific hash value
HASHROW 32 bits of row hash ID without the uniqueness field

HASHAMP Example

If you suspect distribution problems (skewing) among AMPS, the following is a sample of what you might enter for a three-column PI:

SELECT HASHAMP (HASHBUCKET (HASHROW (col_x, col_y, 
      col_z))), count (*)
FROM hash15 
GROUP BY 1
ORDER BY 2 desc;

HASHROW Example

If you suspect collisions in a row hash, the following is a sample of what you might enter for a three-column PI:

SELECT HASHROW (col_x, col_y, col_z), count (*)
FROM hash15 
GROUP BY 1
ORDER BY 2 desc
HAVING count(*) > 10;