Finding Uneven Distribution Using Hash Functions - Teradata Database

Finding Uneven Distribution Using Hash Functions

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;