[Elasticsearch] Function score query

By reading the official document of the Function score query, I still couldn’t get a sense of how to use it correctly. After reading through more articles written by others, here’s how I interpret it with the formula of the newly computed score:

function_score = min(score_mode(f1_score, f2_score, ...), max_boost)
_score = boost_mode(boost*query_score, function_score)

Example 1:

{
  "query": {
      "function_score": {
        "query": { "match_all": {} },
        "boost": "5",
        "random_score": {}, 
        "boost_mode": "multiply"
      }
   }
}

Score:

function_score = min(random_score, FLT_MAX)
_score = 5*1 * function_score
_score = 5*1 * [0..1]

1. Since we didn’t specify the max_boost so the default value will be FLT_MAX.

Example 2:

{
  "query": {
    "function_score": 
      "query": {
        "match_all": {}
      },
      "boost": "5",
      "functions": [
        {
          "filter": {
            "match": {
              "test": "bar"
            }
          },
          "random_score": {},
          "weight": 23
        },    
        {
          "filter": {
            "match": {
              "test": "cat"
            }
          },
          "weight": 42
        }
      ],
      "max_boost": 42,
      "score_mode": "sum",
      "boost_mode": "multiply",
      "min_score": 42
    }
  }
}

Score:

function_score = min(weight1*random_score*ifMatch + weight2*ifMatch, 42)
_score = 5*1 * function_score
_score = 5*1 * (23*random_score*ifMatch + 42*ifMatch)

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