Covering Index

Covering Index
Everyone is familiar with Clustered and non clustered index. In SQL Server 2005 there is an additional keyword provided for the non clustered index called Covering Index. Covering Index is an Index which is only applicable for non clustered Index, which will cover all the columns (columns mentioned in the query eg., Select col1,col2... from T1) in the Index itself. Actually it will improve the perfomance of the query by keeping all the columns in the Index lookup itself.

In the absence of the Covering Index, the Index will have only the Index column (only one column col1). Inorder to fetch other columns again it will use the Index column col1 lookup. This lookup search step (for other columns) you can avoid when you start using the covering Index.

I hope its helps..

Thiyagarajan