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At the back of this book is an index If you were trying to look up information on stored procedures, you could turn to the index and it would tell you on which pages to find that information
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But what if the book had no index We d have to start on page 1 and continue through each page until we found stored procedures mentioned However, if we found stored procedures mentioned on page 44, would we be done Not at all They could also be mentioned in several other places throughout the book, so we would need to check each page until we reached the end However, with an index, we would know exactly which pages have information on stored procedures and we could go directly to those pages Similarly, indexes in databases help the query optimizer find information The query optimizer (sometimes called the query engine) shown in Figure 7-1, does a great deal under the hood in SQL Server When a query is submitted to a database, the query optimizer parses it (makes sure it s syntactically correct), resolves it (makes sure database objects exist), and then optimizes it A significant part of the optimization process is to determine if indexes exist that would relate to the query and decide if they are useful
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Query optimizer Database
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Figure 7-1 The query optimizer
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For example, consider a parts table of over five million parts and the requirement to find the price of a blue widget with a part ID of 101 The following query would be able to find this information quite quickly:
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SELECT Price from Parts WHERE PartID = 101;
The query optimizer would check for indexes on the referenced columns (Price and PartID) in the referenced table (Parts) The PartID column is the PK and, by default, PKs are created with a clustered index The query optimizer would determine that the index on the PartID column would help it find the data the quickest and so would include it in the query plan EXAM TIP Adding indexes to frequently queried data will significantly improve the performance of the queries that use that data When tasked with improving query performance, investigate the possibility of adding indexes to frequently queried data
7: Optimizing Databases
On the other hand, let s say we don t know the part ID of our blue widget We could then use the following query:
SELECT Price from Parts WHERE Name = 'Blue Widget';
If there is no index on the Name column, the query optimizer would have to check all five million rows of the table This is referred to as a table scan and obviously is much slower than the first example where we would use an index SQL Server supports two primary types of indexes that we ll discuss in this section: clustered and nonclustered The absence of an index on a column is referred to as a heap In chapter 5 we presented XML indexes, and later in this chapter we ll present full-text indexes
Clustered
A clustered index is similar to a dictionary or a phone book The data is ordered in the same way the index is ordered For example, if you were to look up the word success in the dictionary, once you reached the word, you would also be on the same page as the definition of the word You wouldn t need to look further The white pages of a phone book work the same way Thinking about a phone book helps to emphasize that we can only order the book in one way (or at least only one way at a time) If you were looking up Ms Olive Yewe, once you found her you would also have her address and phone number: Yewe, Olive 476 Lovers Lane 555-5555 Many years ago, I sold real estate for a short time I worked in an office with several salespeople and we were assigned territories We had a special phone book that was ordered by street names, and it would give the name of who lived there and their phone number I was able to do mass mailings with this information, or if I was feeling especially masochistic, I could try to call everyone on a street in my territory The key point here is that it was a completely separate phone book, ordered in a completely different way In other words, we can t have one physical phone book that is simultaneously organized by both last name and street name Similarly, we can have only one clustered index on a table One physical table can t be ordered in two different ways simultaneously It is possible to create a completely separate copy of the table and order it in a different way, but for one table we can only have one clustered index TIP You can have only one clustered index on a table A single table can only be ordered one way at a time A clustered index orders the data within a table in the same order as the clustered index
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