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  • Is Distinct Aggregation Still Considered Harmful?

    Back in 2008, Marc Friedman of the SQL Server Query Processor Team wrote a blog entry entitled “Distinct Aggregation Considered Harmful”, in which he shows a way to work around the poor performance that often results simply from adding the keyword DISTINCT to an otherwise perfectly reasonable aggregate function in a query.  This post is an ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on December 3, 2011
  • Ranking Function Optimizer Transformations

    In my last post I showed how SQL Server 2005 and later can use a Segment Spool to efficiently implement aggregate window functions and the NTILE ranking function. The query optimizer is also smart enough to recognise that some queries are logically equivalent to a window function, even if they are written using different syntax.  As a ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on July 28, 2010
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