[[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 based on the min/max range of values.

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:

[horizontal]
`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` (default).

Alternatively the `fields` option can also be defined in the request body:

[source,js]
--------------------------------------------------
curl -XPOST "http://localhost:9200/_field_stats?level=indices" -d '{
   "fields" : ["rating"]
}'
--------------------------------------------------

This is equivalent to the previous request.

[float]
=== 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:

[horizontal]
`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: Documents marked as deleted (but not yet removed by the merge process)
still affect all the mentioned statistics.

[float]
=== Cluster level field statistics example

Request:

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

Response:

[source,json]
--------------------------------------------------
{
   "_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.

[float]
==== Indices level field statistics example

Request:

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

Response:

[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.

[float]
=== Field stats index constraints

Field stats index constraints allows to omit all field stats for indices that don't match with the constraint. An index
constraint can exclude indices' field stats based on the `min_value` and `max_value` statistic. This option is only
useful if the `level` option is set to `indices`.

For example index constraints can be useful to find out the min and max value of a particular property of your data in
a time based scenario. The following request only returns field stats for the `answer_count` property for indices
holding questions created in the year 2014:

[source,js]
--------------------------------------------------
curl -XPOST "http://localhost:9200/_field_stats?level=indices" -d '{
   "fields" : ["answer_count"] <1>
   "index_constraints" : { <2>
      "creation_date" : { <3>
         "min_value" : { <4>
            "gte" : "2014-01-01T00:00:00.000Z",
         },
         "max_value" : {
            "lt" : "2015-01-01T00:00:00.000Z"
         }
      }
   }
}'
--------------------------------------------------

<1> The fields to compute and return field stats for.
<2> The set index constraints. Note that index constrains can be defined for fields that aren't defined in the `fields` option.
<3> Index constraints for the field `creation_date`.
<4> An index constraint on the `min_value` property of a field statistic.

For a field, index constraints can be defined on the `min_value` statistic, `max_value` statistic or both.
Each index constraint support the following comparisons:

[horizontal]
`gte`:: 	Greater-than or equal to
`gt`::  	Greater-than
`lte`:: 	Less-than or equal to
`lt`::  	Less-than
