Using Indexes

An index provides fast access to rows of a table based on the values of one or more columns. An index greatly speeds the execution of SQL statements with search conditions that refer to the indexed columns.

Using indexes is appropriate for columns that are used frequently in search conditions. Using indexes is also appropriate in tables for which queries are more frequent than inserts and updates.

SQL Server provides two types of indexes: clustered and nonclustered.

• A clustered index is one in which the logical (indexed) order of the key values is the same as the physical order in which the corresponding rows (that contain the key values) exist in the table. A table can contain only one clustered index.
• A nonclustered index is one in which the logical order of the index does not match the physical order of the rows on disk. Clustered indexes usually provide faster access to data than nonclustered indexes.