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Message Posted: Sun, 21 Apr 2002 @ 11:29:19 GMT


     
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Subj:   Re: Nulls in a composite Primary Key deployed as a Primary Index
 
From:   Dieter Nöth

  Can anyone explain how NULL's affect the hashing power of Teradata? That might be more the concern.  



NULLs should be treated as any other value, too many records with the same RowHash are bad, so there's no difference for storing/joining records if you use 9999 instead of NULL.

But there's one thing in particular: NULLs always hash to '00000000'XB regardless of the datatype, so NULLed rows tend to gather on a single AMP in the system, AFAIK the last AMP in the configuration (probably by design).

Btw, some more values hash to '00000000'XB: Blanks and Zeros, you can do lots of SHL/SHR/AND in a the hash algorithm when the input value has no bits set ;-)


Dieter



     
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