[[rank-feature]]
=== Rank feature field type
++++
<titleabbrev>Rank feature</titleabbrev>
++++

A `rank_feature` field can index numbers so that they can later be used to boost
documents in queries with a <<query-dsl-rank-feature-query,`rank_feature`>> query.

[source,console]
--------------------------------------------------
PUT my-index-000001
{
  "mappings": {
    "properties": {
      "pagerank": {
        "type": "rank_feature" <1>
      },
      "url_length": {
        "type": "rank_feature",
        "positive_score_impact": false <2>
      }
    }
  }
}

PUT my-index-000001/_doc/1
{
  "pagerank": 8,
  "url_length": 22
}

GET my-index-000001/_search
{
  "query": {
    "rank_feature": {
      "field": "pagerank"
    }
  }
}
--------------------------------------------------

<1> Rank feature fields must use the `rank_feature` field type
<2> Rank features that correlate negatively with the score need to declare it

NOTE: `rank_feature` fields only support single-valued fields and strictly positive
values. Multi-valued fields and negative values will be rejected.

NOTE: `rank_feature` fields do not support querying, sorting or aggregating. They may
only be used within <<query-dsl-rank-feature-query,`rank_feature`>> queries.

NOTE: `rank_feature` fields only preserve 9 significant bits for the precision, which
translates to a relative error of about 0.4%.

Rank features that correlate negatively with the score should set
`positive_score_impact` to `false` (defaults to `true`). This will be used by
the <<query-dsl-rank-feature-query,`rank_feature`>> query to modify the scoring formula
in such a way that the score decreases with the value of the feature instead of
increasing. For instance in web search, the url length is a commonly used
feature which correlates negatively with scores.
