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Message Posted: Thu, 21 Aug 2008 @ 15:41:50 GMT


     
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Subj:   Length of Primary Index Columns
 
From:   Goli, Srikanth

Hi -

Does any body know the number of columns or combined length of the columns in Primary Index, that will be considered for hashing the rows. Assume that a Primary Index has 4 columns and they combine to 400 bytes, will all the 400 bytes used in hashing or is there a theoritical limit, say like 256 bytes, after which they will be ignored? ( For the moment ignore how bad a 400 bytes primary index can be, I just used that as an example.) If it does consider only first 256 bytes or so, does it mean that all those rows whose first 256 bytes are same will be hashed to the same AMP? In such case would you order the index columns in such a way that the columns with more unique values are at the beginning of the index, so they are considered for hashing? Does that mean ordering of columns in an index is important?


Thanks.



     
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