doc_values

Most fields are indexed by default, which makes them searchable. The inverted index allows queries to look up the search term in unique sorted list of terms, and from that immediately have access to the list of documents that contain the term.

Sorting, aggregations, and access to field values in scripts requires a different data access pattern. Instead of looking up the term and finding documents, we need to be able to look up the document and find the terms that it has in a field.

The doc_values field is an on-disk data structure that is built at document index time and enables efficient data access. It stores the same values as _source, but in a columnar format that is more efficient for sorting and aggregation.

Doc values are supported on most field types, excluding text and annotated_text fields. See also Disabling doc values.

Numeric types, date types, the boolean type, ip type, geo_point type and the keyword type can also be queried when they are not indexed but only have doc values enabled. Query performance on doc values is much slower than on index structures, but offers an interesting tradeoff between disk usage and query performance for fields that are only rarely queried and where query performance is not as important. This makes doc-value-only fields a good fit for fields that are not expected to be normally used for filtering, for example gauges or counters on metric data.

Doc-value-only fields can be configured as follows:

 PUT my-index-000001 {
  "mappings": {
    "properties": {
      "status_code": { 1
        "type":  "long"
      },
      "session_id": { 2
        "type":  "long",
        "index": false
      }
    }
  }
}

1. The status_code field is a regular long field.
2. The session_id field has index disabled, and is therefore a doc-value-only long field as doc values are enabled by default.

For all fields that support them, doc_values are enabled by default. If you're certain you don't need to sort or aggregate on a field, or access its value from a script, you can disable doc_values in order to save disk space.

In the following example, doc_values is disabled on one field:

 PUT my-index-000001 {
  "mappings": {
    "properties": {
      "status_code": { 1
        "type":       "keyword"
      },
      "session_id": { 2
        "type":       "keyword",
        "doc_values": false
      }
    }
  }
}

1. The status_code field has doc_values enabled by default.
2. The session_id has doc_values disabled, but can still be queried.