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Showing page 2 of 2 (15 total posts)
  • A Tale of Two Index Hints

    If you look up Table Hints in Books Online, you’ll find the following statement: If a clustered index exists, INDEX(0) forces a clustered index scan and INDEX(1) forces a clustered index scan or seek. If no clustered index exists, INDEX(0) forces a table scan and INDEX(1) is interpreted as an error. The interesting thing there is ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on September 22, 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
  • The Impact of Non-Updating Updates

    From time to time, I encounter a system design that always issues an UPDATE against the database after a user has finished working with a record – without checking to see if any of the data was in fact altered.  The prevailing wisdom seems to be that “the database will sort it out”.  This raises an interesting question: how smart is SQL ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on August 10, 2010
  • Iterators, Query Plans, and Why They Run Backwards

    Iterators SQL Server uses an extensible architecture for query optimisation and execution, using ‘iterators’ as basic building-blocks.  Iterators are probably most familiar in their graphical showplan representation, where each icon represents a single iterator.  They also show up in XML query plan output as RelOp nodes. Each ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on August 4, 2010
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