Aggregation query: get value from the latest doc

Elastic Stack Elasticsearch
1

I want to build a query to get the price from the latest document (time). And within the query, there can be other aggregations like “avg”.
The query will be like:

 "query" : {
    // some filters
    "aggregations" : {
           "AVG_price” : {
              "avg" : {
                    "field" : “price”
                 }
              },
               "LAST_price” : {
                  // get the price from the latest, i.e, "price" from the doc of max(“time”); how??
               }
      // snip

Documents have fields like:

“price” : 100
“time” (epoch) : 1531332965000

What will be the best way to have “Last_price”?
We are using Elasitcsearch 5.6.4.
Thanks in advance.

2

Solved with a script. Let me know if you can think of a better solution (especially performance wise).

    "LAST_Price" : {
      "scripted_metric" : {
        "init_script" : {
          "source" : "params._agg.x=0; params._agg.y=\"\";",
          "lang" : "painless"
        },
        "map_script" : {
          "source" : "if (params['_source']['price'] != null) {if (params['_source']['time'] > params._agg.x) {params._agg.x=params['_source']['time'];params._agg.y=params['_source']['price'] }}",
          "lang" : "painless"
        },
        "combine_script" : {
          "source" : "return params._agg",
          "lang" : "painless"
        },
        "reduce_script" : {
          "source" : "long xx=0; String yy=\"\";for (tt in params._aggs) {if (tt.x>xx) {xx=tt.x; yy=tt.y}}return yy",
          "lang" : "painless"
        }
      }
    }
3

What about using a nested top_hits aggregation to return 1 document sorting by age descending.

4

Top_hits works for me; even though I need to work on different result output formats the performance is so much better than using scripts.
Thanks much!!