This functionality is experimental and may be changed or removed completely in a future release. Elastic will take a best effort approach to fix any issues, but experimental features 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:
GET _field_stats?fields=rating
Specific indices:
GET twitter/_field_stats?fields=rating
Supported request options:

A list of fields to compute stats for. The field name supports wildcard notation. For example, using 

Defines if field stats should be returned on a per index level or on a
cluster wide level. Valid values are 
Alternatively the fields
option can also be defined in the request body:
POST _field_stats?level=indices { "fields" : ["rating"] }
This is equivalent to the previous request.
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:

The total number of documents. 

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. 

The percentage of documents that have at least one value for this field. This
is a derived statistic and is based on the 

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. 

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. 
is_searchable
True if any of the instances of the field is searchable, false otherwise.
is_aggregatable
True if any of the instances of the field is aggregatable, false otherwise.

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
Request:
GET _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": "20080801T16:37:51.513Z", "max_value": "20130602T03:23:11.593Z", "is_searchable": "true", "is_aggregatable": "true" }, "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", "is_searchable": "true", "is_aggregatable": "false" }, "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, "is_searchable": "true", "is_aggregatable": "true" }, "rating": { "max_doc": 1326564, "doc_count": 437892, "density": 33, "sum_doc_freq": 1751568, "sum_total_term_freq": 1, "min_value": 14, "max_value": 1277, "is_searchable": "true", "is_aggregatable": "true" } } } } }
When using the cluster level field statistics it is possible to have conflicts if the same field is used in
different indices with incompatible types. For instance a field of type long
is not compatible with a field of
type float
or string
. A section named conflicts
is added to the response if one or more conflicts are raised.
It contains all the fields with conflicts and the reason of the incompatibility.
{ "_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": "20080801T16:37:51.513Z", "max_value": "20130602T03:23:11.593Z", "is_searchable": "true", "is_aggregatable": "true" } } } }, "conflicts": { "field_name_in_conflict1": "reason1", "field_name_in_conflict2": "reason2" } }
Indices level field statistics example
Request:
GET _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": "20080801T16:37:51.513Z", "max_value": "20130602T03:23:11.593Z", "is_searchable": "true", "is_aggregatable": "true" }, "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", "is_searchable": "true", "is_aggregatable": "false" }, "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, "is_searchable": "true", "is_aggregatable": "true" }, "rating": { "max_doc": 1326564, "doc_count": 437892, "density": 33, "sum_doc_freq": 1751568, "sum_total_term_freq": 1, "min_value": 14, "max_value": 1277, "is_searchable": "true", "is_aggregatable": "true" } } } } }
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
. Fields that are not indexed (not searchable) are always omitted when an index constraint is defined.
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:
POST _field_stats?level=indices { "fields" : ["answer_count"], "index_constraints" : { "creation_date" : { "max_value" : { "gte" : "20140101T00:00:00.000Z" }, "min_value" : { "lt" : "20150101T00:00:00.000Z" } } } }
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 

Index constraints for the field 

Index constraints on the 
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:

Greaterthan or equal to 

Greaterthan 

Lessthan or equal to 

Lessthan 
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.