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You searched for the word(s): parallelism
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  • Myth: SQL Server Caches a Serial Plan with every Parallel Plan

    Many people believe that whenever SQL Server creates an execution plan that uses parallelism, an alternative serial plan is also cached.  The idea seems to be that the execution engine then decides between the parallel and serial alternatives at runtime. I’ve seen this on forums, in blogs, and even in books.  In fairness, a lot of the ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on November 4, 2010
  • Forcing a Parallel Query Execution Plan

    This post is for SQL Server developers who have experienced the special kind of frustration, which only comes from spending hours trying to convince the query optimizer to generate a parallel execution plan.  This situation often occurs when making an apparently innocuous change to the text of a moderately complex query; a change which ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on December 22, 2011
  • How Parallelism Works in SQL Server

    You might have noticed that January was a quiet blogging month for me.  Part of the reason was that I was working on a series of articles for Simple Talk, examining how parallel query execution really works.  The first part is published today ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on March 5, 2011
  • Parallel Execution Plans Suck

    Summary: A deep dive into SQL Server parallelism, and a potential performance problem with parallel plans that use TOP. There was an interesting question asked by Mark Storey-Smith on dba.stackexchange.com back in October 2011.  He was looking at the execution plan for a query that counts a million rows from a virtual auxiliary table of ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on May 2, 2012
  • Improving Partitioned Table Join Performance

    The query optimizer does not always choose an optimal strategy when joining partitioned tables. This post looks at an example, showing how a manual rewrite of the query can almost double performance, while reducing the memory grant to almost nothing. Test Data The two tables in this example use a common partitioning partition scheme. The ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on June 16, 2013
  • Is Distinct Aggregation Still Considered Harmful?

    Back in 2008, Marc Friedman of the SQL Server Query Processor Team wrote a blog entry entitled “Distinct Aggregation Considered Harmful”, in which he shows a way to work around the poor performance that often results simply from adding the keyword DISTINCT to an otherwise perfectly reasonable aggregate function in a query.  This post is an ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on December 3, 2011
  • Advanced TSQL Tuning: Why Internals Knowledge Matters

    There is much more to query tuning than reducing logical reads and adding covering nonclustered indexes.  Query tuning is not complete as soon as the query returns results quickly in the development or test environments.  In production, your query will compete for memory, CPU, locks, I/O and other resources on the server.  Today’s ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on February 22, 2011
  • Can a SELECT query cause page splits?

    Books Online has this to say about page splits: When a new row is added to a full index page, the Database Engine moves approximately half the rows to a new page to make room for the new row.  This reorganization is known as a page split.  A page split makes room for new records, but can take time to perform and is a resource ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on August 29, 2011
  • Query Optimizer Deep Dive - Part 1

    This is the first in a series of posts based on the content of the Query Optimizer Deep Dive presentations I have given over the last month or so at the Auckland SQL Users’ Group and the SQL Saturday events in Wellington, New Zealand and Adelaide, Australia. Links to other parts of this series: Part 2 Part 3 Part 4 Introduction The motivation ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on April 27, 2012
  • SQL Server Optimizer Bug with JOIN and GROUP BY

    I came across a SQL Server bug recently that made me wonder how on earth I never noticed it before.  As the title of this post suggests, the bug occurs in common JOIN and GROUP BY queries, and while it does not cause incorrect results to be returned, it will often cause a poor query plan to be selected by the optimizer.  If you are just ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on December 5, 2011
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