title: _update_by_query fails to update all documents in ElasticSearch
body: <p>I have over 30 million documents in Elasticsearch (version - 6.3.3), I am trying to add new field to all existing documents and setting the value to 0.</p>
  
  <p>For example: I want to add <code>start</code> field which does not exists previously in <code>Twitter</code> document, and set it's initial value to 0, in all 30 million documents.</p>
  
  <p>In my case I was able to update 4 million only. If I try to check the submitted task with TASK API <code>http://localhost:9200/_task/{taskId}</code>, result from says something like -></p>
  
  <pre><code>{
    "completed": false,
    "task": {
      "node": "Jsecb8kBSdKLC47Q28O6Pg",
      "id": 5968304,
      "type": "transport",
      "action": "indices:data/write/update/byquery",
      "status": {
        "total": 34002005,
        "updated": 3618000,
        "created": 0,
        "deleted": 0,
        "batches": 3619,
        "version_conflicts": 0,
        "noops": 0,
        "retries": {
          "bulk": 0,
          "search": 0
        },
        "throttled_millis": 0,
        "requests_per_second": -1.0,
        "throttled_until_millis": 0
      },
      "description": "update-by-query [Twitter][tweet] updated with Script{type=inline, lang='painless', idOrCode='ctx._source.Twitter.start = 0;', options={}, params={}}",
      "start_time_in_millis": 1574677050104,
      "running_time_in_nanos": 466805438290,
      "cancellable": true,
      "headers": {}
    }
  }
  </code></pre>
  
  <p>The query I am executing against ES , is something like:</p>
  
  <pre><code>curl -XPOST "http://localhost:9200/_update_by_query?wait_for_completion=false&amp;conflicts=proceed" -H 'Content-Type: application/json' -d'
  {
    "script": {
      "source": "ctx._source.Twitter.start = 0;"
    },
    "query": {
      "exists": {
        "field": "Twitter"
      }
    }
  }'
  </code></pre>
  
  <p>Any suggestions would be great, thanks</p>