title: multiple group by in elasticsearch including missing values
body: <p>I'm trying to do a group by in elasticsearch, by multiple fields. I know that nested aggregation exists, but what I want is including in a certain bucket the record for which the field I'm grouping by is empty.</p>
  
  <p>Say that we have this kind of data structure:</p>
  
  <pre><code>SONG_ID | SONG_GENRE | SONG_ARTIST
  </code></pre>
  
  <p>and i want to group by genere, artists.
  I would like to have a group for each possibile combination, i.e</p>
  
  <p>group by genre gives me 5 buckets (if genres are 5) plus the bucket in which there are the songs without a genre. grouping then by artist gives me, for each genre, bucket by artists plus the one with songs without an artist.</p>
  
  <p>Basically, I'd like to have the same results that I have using a group by. Is that even possible?</p>