Index

• An index is a physical structure containing pointers to the data.
  
• Indices are created in an existing table to locate rows more quickly and efficiently.
  
• It is possible to create an index on one or more columns of a table, and each index is given a name.
  
• The users cannot see the indexes; they are just used to speed up queries.
  
• Effective indexes are one of the best ways to improve performance in a database application.
  
• A table scan happens when there is no index available to help a query. In a table scan SQL Server examines every row in the table to satisfy the query results. Table scans are sometimes unavoidable, but on large tables, scans have a terrific impact on performance.
  
• There are two types of indexes:
  

1. Clustered indexes:        They define the physical sorting of a database table's rows in the storage media. For this reason, each database table may have only one clustered index.
   
2. Non-clustered indexes:     They are created outside of the database table and contain a sorted list of references to the table itself.