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  • 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 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 Page Free Space (Weblog) by Paul White on August 18, 2010
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