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  • SQL Server Internals: Nested Loops Prefetching

    Nested loops join query plans can be a lot more interesting (and complicated!) than is commonly realized. One query plan area I get asked about a lot is prefetching. It is not documented in full detail anywhere, so this seems like a good topic to address in a blog post. The examples used in this article are based on questions asked by Adam ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on August 30, 2013
  • Execution Plan Analysis: The Mystery Work Table

    I love SQL Server execution plans. It is often easy to spot the cause of a performance problem just by looking at one. The task is considerably easier if the plan includes run-time information (a so-called ‘actual’ execution plan), but even a compiled plan can be very useful. Nevertheless, there are still times where the execution plan does not ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on March 7, 2013
  • Incorrect Results with Indexed Views

    Summary: If you use MERGE, indexed views and foreign keys, your queries can return incorrect results. Microsoft have released a fix for incorrect results returned when querying an indexed view. The problem applies to: SQL Server 2012 SQL Server 2008 R2 SQL Server 2008 The Knowledge Base article does not go into much detail, or ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on February 5, 2013
  • Optimizing T-SQL queries that change data

    Most tuning efforts for data-changing operations concentrate on the SELECT side of the query plan. Sometimes people will also look at important storage engine considerations like locking and transaction log throughput that can have dramatic effects. As a consequence, a number of common practices have emerged, such as avoiding large numbers of ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on January 25, 2013
  • Deletes that Split Pages and Forwarded Ghosts

    Can DELETE operations cause pages to split?  Yes.  It sounds counter-intuitive on the face of it; deleting rows frees up space on a page, and page splitting occurs when a page needs additional space.  Nevertheless, there are circumstances when deleting rows causes them to expand before they can be deleted.  The mechanism at ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on August 30, 2012
  • Temporary Table Caching Explained

    SQL Server 2005 onward caches temporary tables and table variables referenced in stored procedures for reuse, reducing contention on tempdb allocation structures and catalogue tables.  A number of things can prevent this caching (none of which are allowed when working with table variables): Named constraints (bad idea anyway, since ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on August 16, 2012
  • Temporary Tables in Stored Procedures

    Ask anyone what the primary advantage of temporary tables over table variables is, and the chances are they will say that temporary tables support statistics and table variables do not.  This is true, of course; even the indexes that enforce PRIMARY KEY and UNIQUE constraints on table variables do not have populated statistics associated with ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on August 14, 2012
  • Query Optimizer Deep Dive - Part 4

    This is the final part 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 1 Part 2 Part 3 Beating the Optimizer ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on April 30, 2012
  • Query Optimizer Deep Dive – Part 3

    This is the third part 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 1 Part 2 Part 4 Storage of Alternative ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on April 29, 2012
  • Query Optimizer Deep Dive – Part 2

    This is the second part 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 1 Part 3 Part 4 Cost-Based Optimization ...
    Posted to Paul White: Page Free Space (Weblog) by Paul White on April 28, 2012
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