I can't imagine most web developers understand or care about or even need to know the intricacies of how a B-Tree or the various types of inverted indexes - or

I can't imagine most web developers understand or care about or even need to know the intricacies of how a B-Tree or the various types of inverted indexes - or other indexes - work. Much more interesting, IMO, and only for the certain type of dev that actually hits the database, is whether they understand that there are different index types, which to use when, and how this know-how translates to them being able to anticipate what the query plan for this or that query will look like based on the available indexes and some pointers on how the data is distributed in disk pages. If anything you want the candidate that, given a set of assumptions, will tell you "a new index is needed here" or "a new index isn't useful for this query" because reasons and is able to explain why in sensible terms. If you're trying to test SQL skill level, take a cursory look at the Postgres tag in StackOverflow and scan for performance-related questions. You'll find plenty of concrete "exercise" types of interview questions that, for the most part, will be straightforward for anyone with the relevant know-how to answer.