title: Adding default value to existing mapping in elastic search
body: <p>I have an index with mapping. I decided to add a new field to existing mapping:</p>
  
  <pre><code>{
  
    "properties": {
      "sexifield": {
        "type": "keyword",
        "null_value": "NULL"
      }
    }
  
  }
  </code></pre>
  
  <p>As far as I understand, the field should appear in existing documents when I reindex. So when I use api to reindex:</p>
  
  <pre><code>{
    "source": {
      "index": "index_v1"
    },
    "dest": {
      "index": "index_v2",
      "version_type": "external"
    }
  }
  </code></pre>
  
  <p>I see that the mapping for index_v2 does not consist sexifield, and documents are not consisting it neither. Also this operation took less than 60ms.</p>
  
  <p>Please point me, what I do not understand from it...</p>
  
  <p>Adding the new documents to the first index (via java API, for an entity which has not this field (sexifield), so probably elastic should add me the default one) with sexifield, also does not create me this additional field.</p>
  
  <p>Thanks in advance for tips.</p>
  
  <p>Regards</p>