Figure 4-20 The Query Optimizer in .NET framework

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Figure 4-20 The Query Optimizer
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In the optimization phase, the Query Optimizer analyzes the query against the existing database, including tables and indexes It tries to determine the most efficient method to retrieve the data While we ll explore indexes in much more depth in 7, let s see a little introduction here As an example, let s say you were looking for information on XML You thought there was information in this book, but you weren t sure One method would be to start at page 1 and look on every page until you came to a topic on XML You d find some information in 2, but would you be done No There s more information on XML, so you would have to continue looking at every page in the book However, it s a common practice to put an index in the back of the book There you could look up XML and find all the places within the book where XML topics are discussed The index makes it much more efficient when searching for a topic That s also how indexes are used within a database Let s say a table within a database takes up 2MB of space An index may take up only 60KB of space It s obviously quicker to search 60KB than to search 2MB The difference is that a table can have multiple indexes The Query Optimizer determines which indexes are most appropriate for the query and then includes them in the query plan The Query Optimizer does a great job of quickly identifying the best way to run a query, but it hasn t always been this way In SQL Server 65 the Query Optimizer left a lot to be desired Database administrators then had to help out the Query Optimizer by telling it what index or indexes to use In other words, DBAs had to provide hints Hints still exist, though today it s quite difficult to consistently come up with a better plan than the Query Optimizer Hints can be provided for queries, for joins, and for tables We ll limit our discussion to query hints
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Query hints are used to direct the Query Optimizer to use a specific index Most queries can be run by the Query Optimizer without any hints More, Microsoft generally recommends against using hints Consider a query that is heavily tested on January 5, when it is determined that it runs best using a specific index By giving the Query Optimizer the hint to use this index, it no longer has to analyze the script and available indexes However, on January 15, as data and indexes have changed, the chosen index is no longer the best choice Since the hint exists, the Query Optimizer no longer analyzes the query, and we live with a poorly performing query In contrast, if hints aren t added to queries, the Query Optimizer continuously analyzes the queries against existing indexes and picks the correct indexes at any given time Today it may pick one index, but as the database dynamically changes, it may pick another index tomorrow
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A hint embedded into a query to direct it to use a specific index that exists for a table may look like the following script The query hint is in bold and directs the Query Optimizer to use the index on the Title column (0 would be Author, 1 is Title, 2 is Price, etc)
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SELECT Author, Title, Price, PublishDate FROM Books WITH (INDEX(1)) WHERE PublishDate > '2001'
EXAM TIP Microsoft has spent a lot of time and effort improving the Query Optimizer with every release of SQL Server Every time a DBA is forced to use a query hint, it s like a slap in the face to the Microsoft SQL Server developers Well, maybe not that bad, but you get the idea If you see a query hint in a query, Microsoft expects you to be able to take it out and have the query run at least as well (if not better) without the query hint than it ran with it
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