The index is one of the main ways to access the high performance of the SQL Server database. It is a database object used to accelerate the polling process by providing quick access to the rows of the database tables. By using indexes, we can save time and improve the performance of database queries and applications. An index contains keys constructed from one or more table columns associated with the specified data storage path. When we create an index in any column, the SQL server internally manages a separate table called an index table, so that each time a user tries to retrieve data from the existing table, the SQL server directly accesses the table of requested data very quickly. In the table, we can use a maximum of 250 indexes. The type of index refers to how the index is stored internally by the SQL server.

So a Table can contain two types of indexes:
  • Clustered Index
  • Non-clustered Index

Explain more about the use of Indexes here.