title: Why are documents in elasticsearch immutable?
body: <p>I am trying to get a hang of elasticsearch. Was reading through the <a href="http://www.elastic.co/guide/en/elasticsearch/guide/current/index.html" rel="nofollow">definitive guide</a>.</p>
  
  <p>They've mentioned that the update API does a retrieve-change-reindex cycle 
  each time I update something in a document. And I completely get that this is done because they say that "Documents are immutable"(see <a href="http://www.elastic.co/guide/en/elasticsearch/guide/current/update-doc.html" rel="nofollow">this</a>). What I am questioning here is why have it immutable in the first place. Wouldn't there be an advantage of allowing update and index of just a particular field hadn't this been the constraint?</p>