Personally, I'd want to have a chat with the interviewer. Most of the time, and by that I mean 99.several nines, you have something wrong in the query that is preventing index use, or the stats are messed up enough that the index is not looking useful, or the index isn't really all that useful. For example, I've seen people who were getting a clustered index scan and insisted that the nonclustered index should have worked fine, so they put a hint in to force the use of the nonclustered index. Now, performance improved, a little, but only because instead of scanning the cluster, it scanned the index which had fewer pages. In fact they didn't fix performance in any way with the hint, just slightly propped it up.