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  • Sorting, Row Goals, and the TOP 100 Problem

    When you write a query to return the first few rows from a potential result set, you’ll often use the TOP clause.  To give a precise meaning to the TOP operation, it will normally be accompanied by an ORDER BY clause.  Together, the TOP…ORDER BY construction can be used to precisely identify which top ‘n’ rows should be returned. The ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on August 26, 2010
  • Row Goals and Grouping

    You might recall (from my last post) that query plans containing a row goal tend to favour nested loops or merge join over hashing.  This is because a hash join has to fully process its build input (to populate its hash table) before it can start probing for matches from its second input.  Hash join therefore has a high start-up cost, ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on August 22, 2010
  • Inside the Optimizer: Row Goals In Depth

    Background One of the core assumptions made by the SQL Server query optimiser’s model is that clients will consume all of the rows produced by a query.  This results in plans that favour the overall execution cost, though it may take longer to begin producing rows.  Let’s look at an example: The optimiser chooses to perform the ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on August 18, 2010
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