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  • re: MERGE gives better OUTPUT options

    Hi Pete - yes. :)
    Posted to Rob Farley (Weblog) by Rob Farley on March 10, 2015
  • Tuning Parallel Data Warehouse Queries

    Performance tuning in regular SQL Server can be addressed in a number of ways. This can involve looking at whats going on with the disk configuration, the memory configuration, the wait stats, the parallelism settings, indexing, and so much more. But if you have a Parallel Data Warehouse (PDW) environment, then there are a lot of things that are ...
    Posted to Rob Farley (Weblog) by Rob Farley on March 9, 2015
  • re: Turning a Scan into a Seek With A Pointless Predicate

    Hi Sachin, I'm sorry, but you're somewhat wrong. A Scan starts at the beginning of the index (or at the end of it's going backwards) to look for matches. It stops when it stops being asked for more records, or when it has looked through the index. A Seek starts at a particular position and then scans through a particular range. It also stops ...
    Posted to Rob Farley (Weblog) by Rob Farley on March 9, 2015
  • re: Why isn’t my filtered index being used?

    Hi Marcel, Yes (as per my post that I linked to above), but if you use parameters (with OPTION (RECOMPILE)) then the QO needs to come up with a plan that is general and is unlikely to use your filtered index.
    Posted to Rob Farley (Weblog) by Rob Farley on February 24, 2015
  • re: Minimising Data Movement in PDW Using Query Optimisation Techniques

    Hi Davos, It's the Query Optimizer that rewrites the queries. In the example where you can't see the 'Unknown' tables, it's because there has been data movement, and that data is stored in the tempdb tables. I'm saying the view is a good thing. It forces the extra predicates to be used, which contradict with each other on the joins that would ...
    Posted to Rob Farley (Weblog) by Rob Farley on February 23, 2015
  • Why isn’t my filtered index being used?

    Quite often, people have filtered indexes but find theyre not being used as often as theyd like. I was reminded of this recently when I read a good post by Kendra Little from about filtered indexes. In it, Kendra talks about how the WHERE clause of a filtered index allows an IN clause, but not an OR clause (to quote someone from ...
    Posted to Rob Farley (Weblog) by Rob Farley on February 22, 2015
  • re: SQL Injection – the golden rule

    Ah yes. Thanks Rich. :)
    Posted to Rob Farley (Weblog) by Rob Farley on February 20, 2015
  • SQL Injection – the golden rule

    The problem with SQL Injection is that most people dont realise the fundamental concept which makes SQL Injection vulnerability not only easy to spot, but also easy to prevent. And its the thing that SQL Injection has in common with countless other hacking mechanisms that have been around since the early days of computing. The simple truth is ...
    Posted to Rob Farley (Weblog) by Rob Farley on February 9, 2015
  • re: Medians pre-SQL 2012

    Thank you Joe - terrific fun to read this.
    Posted to Rob Farley (Weblog) by Rob Farley on February 1, 2015
  • Medians pre-SQL 2012

    SQL 2012 was a big release for working out the median in SQL Server, with the advent of the function PERCENTILE_CONT(). Its a very elegant way of working out the median (hint, thats the 0.5 point), even though its not actually an aggregate function, as Ive written before. Plus it doesnt even perform well. About a year ago, Aaron Bertrand ...
    Posted to Rob Farley (Weblog) by Rob Farley on January 26, 2015
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