title: Advantage of a unique index in MongoDB
body: <p>I've tried to search through Mongo documentation, but can't really find any details on whether queries on unique indexes will be faster than queries on non-unique indexes (given the same data)</p>
  
  <p>So I understand that a unique index will have high selectivity and good performance. But, given two fields whose concatenation is unique, would a non-unique compound index perform slower than a unique compound index?</p>
  
  <p>I am assuming that unique indexes can slow down inserts as the uniqueness must be verified. But is the read performance improvement of a unique index, if any, really worth it?</p>