Microsoft SQL Server Indexes
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Microsoft SQL Server offers two types of indexes: clustered and non-clustered. Clustered indexes physically sort table data to match their logical order. Non-clustered indexes only order the table data logically. In a database, an index lets you speed queries by setting pointers that allow you to retrieve table data without scanning the entire table. An index can be unique or non-unique.
Microsoft SQL Server creates indexes as B-Trees, which are a series of pointers mapping index pages to their underlying data pages. As tables and, therefore, indexes grow, the number of levels in the B-Tree increases. The B-Tree of a clustered index is shorter than that of a non-clustered index because the leaf level of a clustered index is the data page.
A sound indexing strategy is critical to overall system performance. One pitfall to avoid is placing many indexes on a table without regard for their cumulative cost. Remember that indexes improve read but slow write performance because Microsoft SQL Server must update more information in the system catalog. Consequently, extra indexes can actually slow overall performance if data modification occurs frequently on the table. To determine the efficacy of indexes, you should tune your queries using SHOWPLAN and IO STATISTICS and analyze the selectivity of indexes using DBCC SHOW_STATISTICS.
System indexes and user-defined indexes are handled separately in the Datasource Navigator/Explorer to ensure that system indexes are not accidentally altered or dropped.