If most of your candidates cannot answer the question, but you would still hire them I would drop the question. Knowledge of when to use an index is more valuab

If most of your candidates cannot answer the question, but you would still hire them I would drop the question. Knowledge of when to use an index is more valuable than the specific mechanics of how the index is implemented. When I want to test database knowledge, I usually give the candidate a simple database schema and ask them to write progressively harder SQL queries. Database modeling questions are also good for teasing out the seniority of a candidate as well.