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Indexes are database objects that store a subset of a table s columns. They are used to speed data searches by minimizing the amount of information that must be searched by SQL Server. This works in much the same way as you might use the index of this book. Instead of flipping through all of the pages, you would look for simple keywords in the index. When you find what you re looking for, you re referred to a page number that contains the detailed information. Clearly, searching through the index is much quicker than searching through the entire book page-by-page. Indexes are used to reduce the amount of time it takes SQL Server to find the information you require. For example, suppose you often run queries that search for only employee names. However, your database table for employee information actually contains much more information (for example, address, manager s name, and job title). If an index is placed on the First Name and Last Name columns of this table, SQL Server will not have to search through all the information in the database to gain this information. Instead, it will search the index and find the pertinent rows. A query that refers only to in-
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dexed columns is often called a covered query. If details on those rows are required, the index will point to the appropriate data storage areas. SQL Server 7 supports two types of indexes clustered and nonclustered. Clustered indexes involve the physical ordering of items in a database. This means that related information is physically stored together on the hard disk. By default, SQL Server automatically creates a clustered index on the primary key(s) (described later) when a table is created. Since this type determines the actual order in which pages are stored on the disk, a table can have only one clustered index. Clustered indexes increase performance most when you will be returning an ordered subset of information. For example, if I frequently perform queries that require a range of part numbers, access will be much quicker with a clustered index, since these values are stored near each other and thus will require fewer I/O operations to retrieve. However, in databases that are used mainly for transaction information, clustered indexes can actually decrease performance.
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Nonclustered indexes, in contrast, are separate structures that store information but do not affect the physical storage of data on the disk. A table may have many nonclustered indexes. These indexes actually store links to the information in the covered columns, to speed searches. We ll look more closely at indexes, and their effects on performance, later in this chapter.
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Stored procedures are simply named Transact-SQL statements or transactions that are stored within a database as an object. There are several benefits to using stored procedures versus performing the same queries manually. Perhaps the most important benefit is the dramatic speed increase that stored procedures can cause. Stored procedures execute much more quickly than the same statements that execute on an ad-hoc basis. The main reason for this performance increase is that the SQL Server 7 database engine stores a predetermined optimal data recovery plan in cache memory. This is also one reason that you will get a much quicker response time from a query the second time you run it (provided that the data pages haven t yet expired from the cache due to other activities). There are also security benefits to using stored procedures. Like views, stored procedures can be used to hide from users the underlying database objects that are being affected. This helps you restrict the actions that users can perform directly on database objects, and prevents you from having to rely on (and manage) complicated permissions structures. It also allows for better management of underlying table structures. For example, if you want to change an employee s address information in several tables at once, you can create a single stored procedure that will make sure that all the necessary operations are carried out. Additionally, stored procedures can call other stored procedures, allowing the development of modularly coded business rules and SQL statements.
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