title: In Django, do I need to add a db_index to object_id when using a GenericForeignKey?
body: <p>Django's sample code at <a href="https://docs.djangoproject.com/es/1.9/ref/contrib/contenttypes/" rel="nofollow">https://docs.djangoproject.com/es/1.9/ref/contrib/contenttypes/</a></p>
  
  <pre><code>content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
  object_id = models.PositiveIntegerField()
  content_object = GenericForeignKey('content_type', 'object_id')
  </code></pre>
  
  <p>shows object_id without <code>db_index=True</code></p>
  
  <p>although <code>object_id</code> probably will be used for join operation.  </p>
  
  <p>Is <code>db_index=True</code> assumed because of some hidden code regarding GFK? 
  Or should I include it when I need it?</p>