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Field stats API

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Field stats API

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This functionality is in technical preview and may be changed or removed in a future release. Elastic will work to fix any issues, but features in technical preview are not subject to the support SLA of official GA features.

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:

curl -XGET "http://localhost:9200/_field_stats?fields=rating"

Specific indices:

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` (default).

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

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

This is equivalent to the previous request.

Field statistics

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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.
`min_value_as_string`	The lowest value in the field represented in a displayable form. All fields, but string fields returns this. (since string fields, represent values already as strings)
`max_value`	The highest value in the field.
`max_value_as_string`	The highest value in the field represented in a displayable form. All fields, but string fields returns this. (since string fields, represent values already as strings)

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

Cluster level field statistics example

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Request:

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

Response:

{
   "_shards": {
      "total": 1,
      "successful": 1,
      "failed": 0
   },
   "indices": {
      "_all": { 
         "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
            }
         }
      }
   }
}

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

Indices level field statistics example

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Request:

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

Response:

{
   "_shards": {
      "total": 1,
      "successful": 1,
      "failed": 0
   },
   "indices": {
      "stack": { 
         "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
            }
         }
      }
   }
}

The stack key means it contains all field stats for the stack index.

Field stats index constraints

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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:

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

	The fields to compute and return field stats for.
	The set index constraints. Note that index constrains can be defined for fields that aren’t defined in the `fields` option.
	Index constraints for the field `creation_date`.
	Index constraints on the `max_value` and `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:

`gte`	Greater-than or equal to
`gt`	Greater-than
`lte`	Less-than or equal to
`lt`	Less-than

Field stats index constraints on date fields optionally accept a format option, used to parse the constraint’s value. If missing, the format configured in the field’s mapping is used.

curl -XPOST "http://localhost:9200/_field_stats?level=indices" -d '{
   "fields" : ["answer_count"]
   "index_constraints" : {
      "creation_date" : {
         "max_value" : {
            "gte" : "2014-01-01",
            "format" : "date_optional_time" 
         },
         "min_value" : {
            "lt" : "2015-01-01",
            "format" : "date_optional_time"
         }
      }
   }
}'

Custom date format

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