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Message Posted: Wed, 31 Mar 2004 @ 19:27:43 GMT


     
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Subj:   Selectivity high or low ?
 
From:   Rick Davis

Selectivity = (# of rows returned)/(total # of rows)

The selection criteria that returns a small number of rows is said to be highly selective.

The selection criteria that return a large number of rows are said to have low selectivity.

I'm confused , if TableA has 1 miliion rows , suppose particualr NUSI returns 1000 rows , from the formula above

selectvity = 1000/1000000 = .001 , so this has less selectivity , but low selectivity means returning large #of rows . So .001 selecitivity is high or low ?


Thanks,

Rick



     
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