title: When we should use "db_index=True" in Django?
body: <p>When we should define <code>db_index=True</code> on a model fields ?</p>
  <p>I'm trying to optimize the application &amp; I want to learn more about <code>db_index</code>, in which conditions we should use it ?</p>
  <p>The documentation says that using <code>db_index=True</code> on model fields is used to speed up the lookups with slightly disadvantages with storage and memory.</p>
  <p>Should we use <code>db_index=True</code> only on those fields that have unique values like the primary field <code>id</code> ? <br>
  What happens if we enabled indexing for those fields which are not unique and contains repetitive data ?</p>