Index Covering

Question

Hi My question is about the general database-: What is index covering of a query?

James Tomar · Answer

Hi Arjun Index covering Index covering means that Data can be found only using indexes without touching the tables. It can produce dramatic performance improvements when all columns needed by the query are included in the index. You can create indexes on more than one key. These are called composite indexes . Composite indexes can have up to 31 columns adding up to a maximum 600 bytes. If you create a composite nonclustered index on each column referenced in the query's select list and in any where  having  group by  and order by clauses the query can be satisfied by accessing only the index. Since the leaf level of a nonclustered index or a clustered index on a data-only-locked table contains the key values for each row in a table queries that access only the key values can retrieve the information by using the leaf level of the nonclustered index as if it were the actual table data. This is called index covering. There are two types of index scans that can use an index that covers the query: The matching index scan The nonmatching index scan For more information Plz follow the link below: http://www.devx.com/dbzone/Article/29530 Thanks

Satyapriya Nayak · Answer

Hi Arjun Creating a non-clustered index that contains all the columns used in a SQL query a technique called index covering is a quick and easy solution to many query performance problems. Sometimes just adding a column or two to an index can really boost the query's performance. Refer for more http://www.devx.com/dbzone/Article/29530 Thanks