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原文链接：官方文档

Rank feature query，是用来计算相关性得分的，用于boo query里面的should clause。

Rank feature query必须基于rank_feature类型的field，而rank_feature字段必须基于rank_feature类型。

rank_feature 和 rank_features 是用于存储值的特殊类型字段，主要用于对结果进行评分。rank_feature 和 rank_features 中的值只能是单个正值（不允许有多个值）。 在 rank_features 的情况下，值必须是一个 hash 值，由一个字符串和一个正数值组成。

有一个标志可以改变评分的行为 — positive_score_impact。 该值默认为 true，但如果你希望该特征的值降低分数，你可以将其设置为 false。

mapping与数据的例子：

PUT /test
{
  "mappings": {
    "properties": {
      "pagerank": {
        "type": "rank_feature"
      },
      "url_length": {
        "type": "rank_feature",
        "positive_score_impact": false
      },
      "topics": {
        "type": "rank_features"
      }
    }
  }
}

PUT /test/_doc/1?refresh
{
  "url": "https://en.wikipedia.org/wiki/2016_Summer_Olympics",
  "content": "Rio 2016",
  "pagerank": 50.3,
  "url_length": 42,
  "topics": {
    "sports": 50,
    "brazil": 30
  }
}

GET /test/_search
{
  "query": {
    "bool": {
      "must": [
        {
          "match": {
            "content": "2016"
          }
        }
      ],
      "should": [
        {
          "rank_feature": {
            "field": "pagerank"
          }
        },
        {
          "rank_feature": {
            "field": "url_length",
            "boost": 0.1
          }
        },
        {
          "rank_feature": {
            "field": "topics.sports",
            "boost": 0.4
          }
        }
      ]
    }
  }
}

Top-level parameters for rank_feature

field

(Required, string) rank_feature or rank_features field used to boost relevance scores.

boost

(Optional, float) Floating point number used to decrease or increase relevance scores. Defaults to 1.0.

Boost values are relative to the default value of 1.0. A boost value between 0 and 1.0 decreases the relevance score. A value greater than 1.0 increases the relevance score.

saturation

(Optional, function object) Saturation function used to boost relevance scores based on the value of the rank feature field. If no function is provided, the rank_feature query defaults to the saturation function. See Saturation for more information.

Only one function saturation, log, sigmoid or linear can be provided.

log

(Optional, function object) Logarithmic function used to boost relevance scores based on the value of the rank feature field. See Logarithm for more information.

Only one function saturation, log, sigmoid or linear can be provided.

sigmoid

(Optional, function object) Sigmoid function used to boost relevance scores based on the value of the rank feature field. See Sigmoid for more information.

Only one function saturation, log, sigmoid or linear can be provided.

linear

(Optional, function object) Linear function used to boost relevance scores based on the value of the rank feature field. See Linear for more information.

Only one function saturation, log, sigmoid or linear can be provided.