Index Covering of a query in SQL

Question

Hi Friends I want to know about index Covering of a query in SQL and it is used in SQL ?

Santhosh Kumar Jayaraman · Answer

A covering index is an index that contains all and possibly more the columns you need for your query. For instance this: SELECT * FROM tablename WHERE criteria will typically use indexes to speed up the resolution of which rows to retrieve using criteria  but then it will go to the full table to retrieve the rows. However if the index contained the columns column1 column2 and column3  then this sql: SELECT column1 column2 FROM tablename WHERE criteria and provided that particular index could be used to speed up the resolution of which rows to retrieve the index already contains the values of the columns you're interested in so it won't have to go to the table to retrieve the rows but can produce the results directly from the index. Original thread-http://stackoverflow.com/questions/62137/what-is-a-covered-index Check this article to know more http://www.simple-talk.com/sql/learn-sql-server/using-covering-indexes-to-improve-query-performance/