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  • Beware Sneaky Reads with Unique Indexes

    A few days ago, Sandra Mueller (twitter | blog) asked a question using twitter’s #sqlhelp hash tag: “Might SQL Server retrieve (out-of-row) LOB data from a table, even if the column isn’t referenced in the query?” Leaving aside trivial cases (like selecting a computed column that does reference the LOB data), one might be tempted to say ...
    Posted to Page Free Space (Weblog) by Paul White on December 13, 2010
  • 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 Page Free Space (Weblog) by Paul White on September 22, 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 Page Free Space (Weblog) by Paul White on August 4, 2010
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