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A copy of my content from SQLBlog.com plus occasional new content.
Showing posts with label Merge. Show all posts
Showing posts with label Merge. Show all posts

Thursday, 21 February 2013

Halloween Protection – The Complete Series

Halloween Protection – The Complete Series

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I have written a four-part series for SQLPerformance.com on the Halloween Problem.

Some of you will never have heard about this issue. Those that have might associate it only with T-SQL UPDATE queries. In fact, the Halloween Problem affects execution plans for INSERT, UPDATE, DELETE and MERGE statements.

This is a topic I have been meaning to write about properly for years, ever since I read Craig Freedman’s 2008 blog post on the topic, which ended with the cryptic comment:

“…although I’ve used update statements for all of the examples in this post, some insert and delete statements also require Halloween protection, but I’ll save that topic for a future post.”

That future post never materialized, so I thought I would have a go. The four parts of the series are summarized and linked below, I hope you find the material interesting.

Wednesday, 6 February 2013

Incorrect Results with Indexed Views

Incorrect Results with Indexed Views

If you use MERGE, indexed views and foreign keys, your queries might 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 detail, or provide a reproduction script, but this blog post does.

Monday, 10 December 2012

MERGE Bug with Filtered Indexes

MERGE Bug with Filtered Indexes

A MERGE statement can fail, and incorrectly report a unique key violation when:

  • The target table uses a unique filtered index; and
  • No key column of the filtered index is updated; and
  • A column from the filtering condition is updated; and
  • Transient key violations are possible

Wednesday, 4 August 2010

An Interesting MERGE Bug

An Interesting MERGE Bug

Investigating an optimizer transformation that exposes a bug in SQL Server’s MERGE implementation.