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Message Posted: Thu, 18 Dec 2003 @ 20:00:15 GMT


     
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Subj:   can anyone help me out here
 
From:   Rick Davis

Can any one help me out here to understand the following text in a better way . For example if I have a Table with 1000 rows and unique values on NUSI are 25 . How this is realated to the values below

Using NUSIs

The guiding principle for using NUSIs is that there should be fewer rows that satisfy the NUSI qualification condition than there are data blocks in the table.

Whether the Optimizer uses a NUSI depends on the percent of rows per NUSI value, as follows:

IF this many rows

qualify... THEN...

< 1 per block NUSI access is faster than full table scan. For example, if there are 100 rows per block and 1 in 1000 rows qualify, the Optimizer reads 1 in every 10 blocks. NUSI access is faster.

>=1 per block full table scan is faster than NUSI access. For example, if there are 100 rows per block and 1% of the data qualifies, the Optimizer reads almost every block. Full table scan may be faster.


TIA,

Rick



     
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