title: MongoDB warn about query not using index
body: <p>I have a vague memory that MongoDB will output to its log a message if a query uses fields that are not indexed, however I am unable to reproduce this locally (with an admittedly small dataset) nor have I found any documentation for it.</p>
  
  <p>Am I imagining things or is there such a feature? If yes, can anyone link to documentation?</p>