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Using Distinct or group by

  • April 3, 2009 at 12:45 am

    #132773

    Hi,

    Another day,another query.

    I know that,we should not use distinct clause frequently.

    Because using the DISTINCT clause will result in some performance degradation, you should use this clause only when it is necessary.

    Instead I use Group by clause is it ok?Result are same.

    what about performance ?

    [font="Verdana"]Regards
    Kumar Harsh[/font]

    :unsure:

  • April 3, 2009 at 6:02 am

    #971433

    You can see in the execution plan that both options are executed alike.

    But only use DISTINCT or GROUP BY when required, because they can cause real performance hits.

    ** Don't mistake the ‘stupidity of the crowd’ for the ‘wisdom of the group’! **

  • April 3, 2009 at 6:10 am

    #971441

    DISTINCT and GROUP BY are both methods of aggregating data. In general terms, neither is going to perform better than the other one.

    In general practice, you need to structure your storage such that you don't have lots and lots of duplicate data requiring you to aggregate it in order to return valid result sets. That, or you need to examine your code to ensure you're accessing the data correctly, using all the columns in the key and in foreign keys, etc. It's not that you should only use aggregation sparingly in your system, you should only need to use it sparingly (or specifically when generating aggregation for reports, etc.).

    "The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
    - Theodore Roosevelt

    Author of:
    SQL Server Execution Plans
    SQL Server Query Performance Tuning

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