How to subtract aggregate min from aggreagate max(difference) in ES?

Question

How to write an ES query to find the difference between max and min value of a field?

I am a newbee in elastic search, In my case I feed lot of events along with session_id and time in to elastic search. My event structure is

Event_name string    `json:"Event_name"`
Client_id  string    `json:"Client_id"`
App_id     string    `json:"App_id"`
Session_id string    `json:"Session_id"`
User_id    string    `json:"User_id"`
Ip_address string    `json:"Ip_address"`
Latitude   int64     `json:"Latitude"`
Longitude  int64     `json:"Longitude"`
Event_time time.Time `json:"Time"`

I want to find the life time of a session_id based the feeded events. For that I can retrive the maximum Event_time and minimum Event_time for a particular session_id by the following ES query.

{  
  "size": 0,
  "query": {
     "match": {
        "Session_id": "dummySessionId"
     }
  },
   "aggs": {
      "max_time": {
         "max": {
           "field": "Time"
          }
       },
       "min_time":{
          "min": {
            "field": "Time"
          }
       }
    }
  }

But what I exact want is (max_time - min_time) How to write the ES query for the same????

Answer 1

Up to elasticsearch 1.1.1, this is not possible to do any arithmetic operation upon two aggregate function's reasult from elasticsearch side. If you want then, you should do that from client side.

That is neither possible through scripts, as @eliasah suggests.

In the upcoming versions they may be add such facility.

Answer 2

in the 1.5.1 using the Scripted Metric Aggregation you can do this. Not sure about the performance, but it looks to work. This functionality is experimental and may be changed or removed completely in a future release.

POST test_time

POST test_time/data/1
{"Session_id":1234,"Event_time":"2014-01-01T12:00:00"}

POST test_time/data/3
{"Session_id":1234,"Event_time":"2014-01-01T14:00:00"}

GET /test_time/_search
{
  "size": 0,
  "aggs": {
    "by_user": {
      "terms": {
        "field": "Session_id"
      },
      "aggs": {
        "session_lenght_sec": {
          "scripted_metric": {
            "map_script": "_agg['v'] = doc['Event_time'].value",
            "reduce_script": "min = null; max = null; for (a in _aggs) {if (min == null || a.v < min) { min = a.v}; if (max == null || a.v > max) { max = a.v }}; return (max-min)/1000"
          }
        }
      }
    }
  }
}

###### RESPONSE #######
{
   ...,
   "aggregations": {
      "by_user": {
         "doc_count_error_upper_bound": 0,
         "sum_other_doc_count": 0,
         "buckets": [
            {
               "key": 1234,
               "doc_count": 2,
               "session_lenght_sec": {
                  "value": "7200"
               }
            }
         ]
      }
   }
}