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.Not to mention a lot of real-world experience intuning applications.You know the story: optimizing a stored procedure that ran in16 hours so that it ran in 20 minutes.In the first edition of this book, I includedsome brief information about some rare cases in index covering you should experimentwith.The tech editors burned me up, complaining that the correct use of covered indexesrequired much more detail and information than I was providing in my brief description.So I decided to check the Microsoft documentation, which also did not favor the useof index covering.Out voted, two to one, I toned down the recommendation of experimentingwith index covering in some critical cases.When the first edition of this book waspublished, everyone liked the book but pointed out that they do not agree with mystand on covered indexes.I even got e-mail from several respected SQL Server expertswho commented on the index covering section.Okay.In this edition, I'm adding my original recommendation that index coveringin many cases can give you better performance than a clustered index when runninga query.But in all honesty, do not go crazy and try to cover every query you execute.Trying to cover every query you execute is a ridiculous thought and will end up bringingyour system performance to a crawl when performing updates, inserts, and deletes.Keep in mind that the optimizer uses only one index.In general, you should stickwith single-column or narrow composite indexes for the query optimizer to use.Butif you still need better performance and are not returning a lot of columns or havea large WHERE clause and the query is of a critical nature, experiment withindex covering.I hope this time around I have covered (no pun intended) all thedifferent viewpoints on this topic!Between the LinesFollowing are some important points to remember about SQL Server indexes:SQL Server maintains indexes with a B-Tree structure.In a clustered index, the leaf contains the actual data pages of the table andthe data is physically stored in the logical order of the index.The SQL Server query optimizer selects at most one index to resolve a query.Composite indexes are indexes created with two or more columns.Select indexes carefully.SummaryIt is important that you understand the basic ideas behind SQL Server indexes.In the next chapter, you build on the basic concepts of this chapter and learn aboutthe query optimizer.If you understand indexes and the query optimizer, you willbe able to provide valuable support to developers.DISCLAIMERTo order books from QUE, call us at 800-716-0044or 317-361-5400.For comments or technical support for our books and software, select Talk to Us.© 1997, QUE Corporation, an imprint of Macmillan Publishing USA, a Simon and SchusterCompany [ Pobierz całość w formacie PDF ]
