You have to be careful what you wish for. The signal:noise ratio of finding out which indexes are “unused” is low. This isn’t obvious at first glance, but upon deeper inspection, there’s often not much to be gained, and it can even harm you. Let’s look at why.
Queries can be slow or resource-heavy for many reasons, but one of the most common is that the table doesn't have a good index for the query. This is true for MySQL, PostgreSQL, and especially for MongoDB. Adding an index where it's missing is often a night-and-day improvement, as shown in this screenshot of a query's performance. The index was added midway through the time range, and the query got dramatically faster: