Home Page for the TeradataForum
 

Archives of the TeradataForum

Message Posted: Sun, 05 Dec 2004 @ 19:19:22 GMT


     
  <Prev Next>   <<First <Prev Next> Last>>  


Subj:   Re: Why is Group-by Faster than Distinct
 
From:   Dieter Noeth

Anomy Anom wrote:

  Is it a standard optimization technique now to use Group by instead of Distinct to optimize queries that need to remove duplicate records?  


No, it depends...

All PI columns -> Group By

Lots of rows per AMP per value -> Group by

Up to a few rows per AMP per value -> Distinct

And remember, that a subquery is distinct by default, but you can override it with group by .


  Is there any difference in results, in case I am going to use group by instead of distinct?  


Distinct returns an ordered result set, group by doesn't.


  If there is no difference why doesn't the optimizer use the duplicate removal operation which is faster in Group by instead of the one that is use in distinct?  


AFAIK there's already an enhancement request for that :-)


Dieter



     
  <Prev Next>   <<First <Prev Next> Last>>  
 
 
 
 
 
 
 
 
  
  Top Home Privacy Feedback  
 
 
Copyright for the TeradataForum (TDATA-L), Manta BlueSky    
Copyright 2016 - All Rights Reserved    
Last Modified: 15 Jun 2023