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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:
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:

A list of fields to compute stats for. 

Defines if field stats should be returned on a per index level or on a
cluster wide level. Valid values are 
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. 

The lowest value in the field represented in a displayable form. 

The highest value in the field represented in a displayable form. 
Documents marked as deleted (but not yet removed by the merge process) still affect all the mentioned statistics.
Example 1. Cluster level field statistics
GET /_field_stats?fields=rating,answer_count,creation_date,display_name
{ "_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" }, "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 } } } } }
Example 2. Indices level field statistics
GET /_field_stats?fields=rating,answer_count,creation_date,display_name&level=indices
{ "_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" }, "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 } } } } }