Add the ability to sort aggregations by allowing arithmetic operations between sub aggregations

[nitzanharel]

I want to get the top X event senders for a time range but sorted not by doc count but rather by percentage increase/decrease to a different time range.
I know I can do this:

  "query": {
    "bool": {
      "must": [
        {
          "range": {
            "sentTimestamp": {
              "gte": "2014-10-07T00:01:00.0Z",
              "lt": "2014-10-07T00:01:50.0Z"
            }
          }
        }
      ]
    }
  },
  "size": 0,
  "aggs": {
    "topSenders": {
      "terms": {
        "field": "sender",
        "size": 5,
        "order": {
          "t2>tu.value": "desc"
        }
      },
      "aggs": {
        "t1": {
          "filter": {
            "range": {
              "sentTimestamp": {
                "gte": "2014-10-07T00:01:00.0Z",
                "lt": "2014-10-07T00:01:40.0Z"
              }
            }
          },
          "aggs": {
            "tu": {
              "cardinality": {
                "field": "sender"
              }
            }
          }
        },
        "t2": {
          "filter": {
            "range": {
              "sentTimestamp": {
                "gte": "2014-10-07T00:01:40.0Z",
                "lt": "2014-10-07T00:01:50.0Z"
              }
            }
          },
          "aggs": {
            "tu": {
              "cardinality": {
                "field": "sender"
              }
            }
          }
        }
      }
    }
  }
}

You can see I am sorting by "t2>tu.value". If I was able to sort by something like "(t2>tu.value-t2>tu.value)/t1>tu.value" I would be able to get that all in one query

[clintongormley]

This looks like a job for reducers, see #8110.

@colings86 a sorting reducer would be a nice addition.

[clintongormley]

Actually, @colings86 has pointed out that this would be possible with a scripted sort provided by #6917

[clintongormley]

I'm going to close this in favour of #6917