Covered Queries

A nonclustered index covers a query when all columns in the SELECT list and all columns in the WHERE clause are part the nonclustered index. In this case, the optimizer uses the index for data retrieval, because all required data can be read from the index pages, and the data pages do not need to be read. However, avoid creating excessively wide indexes and hoping that the optimizer will use this strategy. It is usually best to use more numerous narrow indexes, which often provides better performance over a wider range of queries.