title: What are the limitations of partial indexes?
body: <p>The latest version of MongoDB (v3.2) adds support for <a href="https://docs.mongodb.org/manual/core/index-partial/" rel="noreferrer">partial (filtered) indexes</a>. You supply a filter when you create the index and that filter determines which documents will be referenced in the index and which will not.</p>
  
  <p>Can I use any filter expression (as long as it's a valid filter)? Or are there limitations to the filter being used? If so, what are those limitations?</p>