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If the seek continues, it will reach the emp_E record again and update it a second time, and of course, that would be incorrect. Now observe the position of the Table Spool in the preceding query planit separates the Clustered Index Seek and Clustered Index Update operators. The spool consumes all records from the index seek before it proceeds with the updates against the same clustered index. The spool, not the index, is producing the updated values. Therefore, the spool prevents updating the same record twice and ensures a correct result. We call such a spool in our IUD query plans a Halloween spool. You might be wondering what Halloween has in common with the query plans just described We have to go back almost 30 years in the history of database technologyand query optimization in particularfor the answer. Researchers at the Almaden Research Center in California encountered the same problem when they trained their query optimizer prototype to use indexes in update plans. In fact, they used the salary column and salary increase update query similarly to what I just did. Moreover, to their big surprise nobody had a salary under 25,000 after they ran the update query! It was on Halloween 1977 (or maybe 1976 ) when the researchers described and debated the glitch. Since then, many database texts, papers, and articles have used the "Halloween" term to name the double-update problem I just outlined.
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Physical query processing consists of two fundamental steps: query compilation and query execution. The main link between the two steps is the query plan. Understanding how SQL Server generates the query plans and how it is using the plans to deliver the query result is vital for application developers, database designers, and administrators. Constructive use of the knowledge of how SQL Server performs its physical query processing might improve the database server's performance and the application's response time and throughput.
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Acknowledgment
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I received great help from several colleagues at Microsoft while I was working on this chapter. Mike Blaszczak, Alexis Boukouvalas, Milind Joshi, Stefano Stefani, Don Vilen, Umachandar Jayachandran, and Eugene Zabokritski reviewed and commented on part of the chapter or on the whole chapter and provided me with a wealth of feedback and ideas for improving the content.
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3. Query Tuning
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In this chapter: Sample Data for This Tuning Methodology Tools for Query Tuning Index Tuning Preparing Sample Data 66 69 105 122 170
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An Examination of Set-Based vs. Iterative/Procedural Approaches, and a Tuning Exercise 180 Additional Resources Conclusion 187 189
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This chapter lays the foundation of query tuning knowledge required for both this book and Inside Microsoft SQL Server 2005: T-SQL Programming. (For brevity, I'll refer to the programming book as Inside T-SQL Programming, and to both this book and Inside T-SQL Programming as "these books.") Here you will be introduced to a tuning methodology, acquire tools for query tuning, learn how to analyze execution plans and perform index tuning, and learn the significance of preparing good sample data and the importance of using set-based solutions. When building the table of contents for this book, I faced quite a dilemma with regard to the query tuning chapter, a dilemma which I've also faced when teaching advanced T-SQLshould this material appear early or late On one hand, the chapter provides important background information that is required for the rest of the book; on the other hand, some techniques used for query tuning involve advanced queriessort of a chicken and egg quandary. I decided to incorporate the chapter early in the book, but I wrote it as an independent unit that can be used as a reference. My recommendation is that you read this chapter before the rest of the book, and when a query uses techniques that you're not familiar with yet, just focus on the conceptual elements described in text. Some queries will use techniques that are described later in the book (for example, pivoting, running aggregations, the OVER clause, CUBE, CTEs, and so on) or in Inside T-SQL Programming (for example, temporary tables, cursors, routines, CLR integration, compilations, and so on). Don't be concerned if the techniques are not clear. Feel free, though, to jump to the relevant chapter if you're curious about a certain technique. When you finish reading the books, I suggest that you return to this chapter and revisit any queries that were not clear at first to make sure you fully understand their mechanics. At the end of this chapter, I'll provide pointers to resources where you can find more information about the subject.
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