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Message Posted: Fri, 11 Feb 2005 @ 17:14:02 GMT


     
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Subj:   Re: Guidelines for using Distinct vs. group by
 
From:   Dieter Noeth

Brian Jones wrote:

  Are there guidelines on when to use distinct vs. group by? Thank you.  


For columns with equal distribution (for skewed distribution it's hard to say):

- Distinct spools and redistibutes all rows, then eliminates duplicate rows -> efficient if the number of rows per value per AMP is low

- Group by eliminates duplicate rows in the first step locally, then redistributes the result sets and eliminates duplicate rows a second time -> efficient if the nuber of rows per value per AMP is high

- If PI-columns are aggregated ("computed locally" in explain) then Group By is always more efficient.

- And don't forget that a subquery is automatically distinct, but using Group By overrides that.


Dieter



     
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