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Message Posted: Thu, 05 Apr 2012 @ 14:31:42 GMT


     
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Subj:   Re: How Hashing algorithm works?
 
From:   Mankala, Mahesh

Hi,

The system uses the Primary Index value in each row as input to the hashing algorithm.

The output of the hashing algorithm is the row hash value .

The system looks at the hash map, which identifies the specific AMP where the row will be stored .

The row is stored on the target AMP.

It is possible for the hashing algorithm to end up with the same row hash value for two different rows. There are two ways this could happen:

Duplicate NUPI values: If a Non-Unique Primary Index is used, duplicate NUPI values will produce the same row hash value.

Hash synonym: Also called a hash collision, this occurs when the hashing algorithm calculates an identical row hash value for two different Primary Index values.

Hash synonyms are rare. When using a Unique Primary Index, you will still get uniform data distribution.

To differentiate each row in a table, every row is assigned a unique Row ID. The Row ID is the combination of the row hash value and a uniqueness value.

Row ID = Row Hash Value + Uniqueness Value

The uniqueness value is used to differentiate between rows whose Primary Index values generate identical row hash values.

In most cases, only the row hash value portion of the Row ID is needed to locate the row.


Thanks,

Mahesh



     
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