query.json

{
   "size": 0,
   "query": {
      "match_all": {}
   },
   "aggs": {
      "browser_distribution": {
         "terms": {
            "field": "browser"
         },
         "aggs": {
            "version_distribution": {
               "terms": {
                  "field": "browser_version"
               }
            }
         }
      },
      "browser_overall_count": {
         "value_count": {
            "field": "browser"
         }
      }
   }
}

response.json

{
   "took": 16,
   "timed_out": false,
   "_shards": {
      "total": 5,
      "successful": 5,
      "failed": 0
   },
   "hits": {
      "total": 6,
      "max_score": 0,
      "hits": []
   },
   "aggregations": {
      "browser_distribution": {
         "buckets": [
            {
               "key": "chrome",
               "doc_count": 3,
               "version_distribution": {
                  "buckets": [
                     {
                        "key": "35.0.1916.153",
                        "doc_count": 3
                     }
                  ]
               }
            },
            {
               "key": "safari",
               "doc_count": 2,
               "version_distribution": {
                  "buckets": [
                     {
                        "key": "5.1.3",
                        "doc_count": 2
                     }
                  ]
               }
            },
            {
               "key": "firefox",
               "doc_count": 1,
               "version_distribution": {
                  "buckets": [
                     {
                        "key": "27.0",
                        "doc_count": 1
                     }
                  ]
               }
            }
         ]
      },
      "browser_overall_count": {
         "value": 6
      }
   }
}