After a few queries regarding my recent post on Covering Indexes, I decided to do a couple of tests to show you the results.
Below are the actual execution plan results using the table structure and SQL query from the earlier post. The table had three rows of data.
As you can (hopefully) see, the covering index resulted in a simpler execution plan.
If we compare the two subtree costs of 0.0032831 (Covering index) and 0.0065704 (No covering index) we get a performance gain of 2.001 or almost bang on 200% improvement.
I hope this helps