I feel the question could have been worded better. Specifically, I am referring to this sentence:
Which one of the following things that is not usually used to improve data warehouse query performance?
That sentence sounds like it's asking: Which one of the following would help query performance but is not usually used?
However, that is not what the question is really asking.
I think removal of the word 'that' in the quoted QotD sentence above would help improve readability.