Archives of the TeradataForum
Message Posted: Sun, 21 Apr 2002 @ 11:29:19 GMT
| 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 ;-)