Jeevan

• Aug 22nd, 2007

 

If all the columns in table on which there is an index are used in the join clause in the same order as it is in the index definition then it is called as index covered.
It helps us to improve the performance.
It is a good practice to include all the columns in the join clause which are present in the index definition

Arun

• Sep 8th, 2007

 

The data that are fetched by the SQL (select statement) is completely fetched from the index itself is called index covering.
Example:
-- TableA has 10 columns Col1, Col2, Col3, Col4, Col5...
-- An index idx_TableA on TableA has composite non-clustered index on columns(Col1, Col2, Col3)
The following SQL statement uses the index idx_TableA. As the index has Col2 and Col3 as well, data will be picked up from the index itself without going to table.
Select Col2, Col3 from TableA
Where Col1 = 'Value1'