SQL indexes are the medium of reducing the cost of the query as the high cost of the query will lead to the fall in the performance of the query. An index is used to increase the performance and allow faster retrieval of records from the table. Indexing reduces the number of data pages we need to visit to find a particular data page. Indexing also has a unique value that means that the index cannot be duplicated. An index creates an entry for each value, and it will be faster to retrieve data. For example, suppose you have a book which carries the details of the countries, and you want to find out the information about India than why you will go through every page of that book you could directly go to the index, and then from index you can go to that particular page where all the information about India is given.