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Message Posted: Thu, 10 Apr 2003 @ 14:52:12 GMT


     
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Subj:   NUSI calculation
 
From:   Rodrigo Cifuentes

I found a formula in Teradata Factory docs (PDBD student manual) to evaluate if a NUSI is appropriate in a column. The formula is

Typicall Rows per Value < (Typical Row Size / Typical Block size) (as a percentage of) Number of Rows in Table.

An example goes like this:

Typical Rows per Value = 2000
Typical Row Size = 480
Typical Block Size = 48000
Number of Rows = 100.000
480/48000 = 1/100 = 1%
1% of 100.000 = 1000

2000 < 1000 is false, so no NUSI should be built on the column.

Anybody knows the reasoning behind the formula? Why bigger rows 'allow' to have more repetitions on the NUSI/primary table? The only reason I can see is that the number of read on NUSI blocks is increasingly favorable when the number of records decreas in the data block. Is this the only reason?



     
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