OpenSearch/docs/reference/aggregations/metrics/valuecount-aggregation.asciidoc

[[search-aggregations-metrics-valuecount-aggregation]]
=== Value Count Aggregation

A `single-value` metrics aggregation that counts the number of values that are extracted from the aggregated documents.
These values can be extracted either from specific fields in the documents, or be generated by a provided script. Typically,
this aggregator will be used in conjunction with other single-value aggregations. For example, when computing the `avg`
one might be interested in the number of values the average is computed over.

`value_count` does not de-duplicate values, so even if a field has duplicates (or a script generates multiple
identical values for a single document), each value will be counted individually.

[source,console]
--------------------------------------------------
POST /sales/_search?size=0
{
    "aggs" : {
        "types_count" : { "value_count" : { "field" : "type" } }
    }
}
--------------------------------------------------
// TEST[setup:sales]

Response:

[source,console-result]
--------------------------------------------------
{
    ...
    "aggregations": {
        "types_count": {
            "value": 7
        }
    }
}
--------------------------------------------------
// TESTRESPONSE[s/\.\.\./"took": $body.took,"timed_out": false,"_shards": $body._shards,"hits": $body.hits,/]

The name of the aggregation (`types_count` above) also serves as the key by which the aggregation result can be
retrieved from the returned response.

==== Script

Counting the values generated by a script:

[source,console]
--------------------------------------------------
POST /sales/_search?size=0
{
    "aggs" : {
        "type_count" : {
            "value_count" : {
                "script" : {
                    "source" : "doc['type'].value"
                }
            }
        }
    }
}
--------------------------------------------------
// TEST[setup:sales]

This will interpret the `script` parameter as an `inline` script with the `painless` script language and no script parameters. To use a stored script use the following syntax:

[source,console]
--------------------------------------------------
POST /sales/_search?size=0
{
    "aggs" : {
        "types_count" : {
            "value_count" : {
                "script" : {
                    "id": "my_script",
                    "params" : {
                        "field" : "type"
                    }
                }
            }
        }
    }
}
--------------------------------------------------
// TEST[setup:sales,stored_example_script]

NOTE:: Because `value_count` is designed to work with any field it internally treats all values as simple bytes.
Due to this implementation, if `_value` script variable is used to fetch a value instead of accessing the field
directly (e.g. a "value script"), the field value will be returned as a string instead of it's native format.

[[search-aggregations-metrics-valuecount-aggregation-histogram-fields]]
==== Histogram fields
When the `value_count` aggregation is computed on <<histogram,histogram fields>>, the result of the aggregation is the sum of all numbers
in the `counts` array of the histogram.

For example, for the following index that stores pre-aggregated histograms with latency metrics for different networks:

[source,console]
--------------------------------------------------
PUT metrics_index/_doc/1
{
  "network.name" : "net-1",
  "latency_histo" : {
      "values" : [0.1, 0.2, 0.3, 0.4, 0.5],
      "counts" : [3, 7, 23, 12, 6] <1>
   }
}

PUT metrics_index/_doc/2
{
  "network.name" : "net-2",
  "latency_histo" : {
      "values" :  [0.1, 0.2, 0.3, 0.4, 0.5],
      "counts" : [8, 17, 8, 7, 6] <1>
   }
}

POST /metrics_index/_search?size=0
{
    "aggs" : {
        "total_requests" : {
            "value_count" : { "field" : "latency_histo" }
        }
    }
}
--------------------------------------------------

For each histogram field the `value_count` aggregation will sum all numbers in the `counts` array <1>.
Eventually, it will add all values for all histograms and return the following result:

[source,console-result]
--------------------------------------------------
{
    ...
    "aggregations" : {
        "total_requests" : {
           "value" : 97
        }
    }
}
--------------------------------------------------
// TESTRESPONSE[skip:test not setup]