[[search-field-stats]]
== Field stats API

experimental[]

The field stats api allows one to find statistical properties of a field without executing a search, but
looking up measurements that are natively available in the Lucene index. This can be useful to explore a dataset which
you don't know much about. For example, this allows creating a histogram aggregation with meaningful intervals.

The field stats api by defaults executes on all indices, but can execute on specific indices too.

All indices:

[source,js]
--------------------------------------------------
curl -XGET "http://localhost:9200/_field_stats?fields=rating"
--------------------------------------------------

Specific indices:

[source,js]
--------------------------------------------------
curl -XGET "http://localhost:9200/index1,index2/_field_stats?fields=rating"
--------------------------------------------------

Supported request options:
* `fields` - A list of fields to compute stats for.
* `level`  - Defines if field stats should be returned on a per index level or on a cluster wide level. Valid values are
             `indices` and `cluster`. Defaults to `cluster`.

=== Field statistics

The field stats api is supported on string based, number based and date based fields and can return the following statistics per field:

* `max_doc`             - The total number of documents.
* `doc_count`           - The number of documents that have at least one term for this field, or -1 if this measurement isn't available on one or more shards.
* `density`             - The percentage of documents that have at least one value for this field. This is a derived statistic and is based on the `max_doc` and `doc_count`.
* `sum_doc_freq`        - The sum of each term's document frequency in this field, or -1 if this measurement isn't available on one or more shards.
                          Document frequency is the number of documents containing a particular term.
* `sum_total_term_freq` - The sum of the term frequencies of all terms in this field across all documents, or -1 if this measurement isn't available on one or more shards.
                          Term frequency is the total number of occurrences of a term in a particular document and field.
* `min_value`           - The lowest value in the field represented in a displayable form.
* `max_value`           - The highest value in the field represented in a displayable form.

Note that for all the mentioned statistics, documents marked as deleted aren't taken into account. The documents marked
as deleted are are only taken into account when the segments these documents reside on are merged away.

=== Example

[source,js]
--------------------------------------------------
curl -XGET "http://localhost:9200/_field_stats?fields=rating,answer_count,creation_date,display_name"
--------------------------------------------------

[source,js]
--------------------------------------------------
{
   "_shards": {
      "total": 1,
      "successful": 1,
      "failed": 0
   },
   "indices": {
      "_all": { <1>
         "fields": {
            "creation_date": {
               "max_doc": 1326564,
               "doc_count": 564633,
               "density": 42,
               "sum_doc_freq": 2258532,
               "sum_total_term_freq": -1,
               "min_value": "2008-08-01T16:37:51.513Z",
               "max_value": "2013-06-02T03:23:11.593Z"
            },
            "display_name": {
               "max_doc": 1326564,
               "doc_count": 126741,
               "density": 9,
               "sum_doc_freq": 166535,
               "sum_total_term_freq": 166616,
               "min_value": "0",
               "max_value": "정혜선"
            },
            "answer_count": {
               "max_doc": 1326564,
               "doc_count": 139885,
               "density": 10,
               "sum_doc_freq": 559540,
               "sum_total_term_freq": -1,
               "min_value": 0,
               "max_value": 160
            },
            "rating": {
               "max_doc": 1326564,
               "doc_count": 437892,
               "density": 33,
               "sum_doc_freq": 1751568,
               "sum_total_term_freq": -1,
               "min_value": -14,
               "max_value": 1277
            }
         }
      }
   }
}
--------------------------------------------------

<1> The `_all` key indicates that it contains the field stats of all indices in the cluster.

With level set to `indices`:

[source,js]
--------------------------------------------------
curl -XGET "http://localhost:9200/_field_stats?fields=rating,answer_count,creation_date,display_name&level=indices"
--------------------------------------------------

[source,js]
--------------------------------------------------
{
   "_shards": {
      "total": 1,
      "successful": 1,
      "failed": 0
   },
   "indices": {
      "stack": { <1>
         "fields": {
            "creation_date": {
               "max_doc": 1326564,
               "doc_count": 564633,
               "density": 42,
               "sum_doc_freq": 2258532,
               "sum_total_term_freq": -1,
               "min_value": "2008-08-01T16:37:51.513Z",
               "max_value": "2013-06-02T03:23:11.593Z"
            },
            "display_name": {
               "max_doc": 1326564,
               "doc_count": 126741,
               "density": 9,
               "sum_doc_freq": 166535,
               "sum_total_term_freq": 166616,
               "min_value": "0",
               "max_value": "정혜선"
            },
            "answer_count": {
               "max_doc": 1326564,
               "doc_count": 139885,
               "density": 10,
               "sum_doc_freq": 559540,
               "sum_total_term_freq": -1,
               "min_value": 0,
               "max_value": 160
            },
            "rating": {
               "max_doc": 1326564,
               "doc_count": 437892,
               "density": 33,
               "sum_doc_freq": 1751568,
               "sum_total_term_freq": -1,
               "min_value": -14,
               "max_value": 1277
            }
         }
      }
   }
}
--------------------------------------------------

<1> The `stack` key means it contains all field stats for the `stack` index.