how does field_value_factor work in elasticsearch?

Question

The documentation says that using a field_value_factor value of:

"field_value_factor": {
  "field": "popularity",
  "factor": 1.2,
  "modifier": "sqrt",
  "missing": 1
}

"Which will translate into the following formula for scoring:

sqrt(1.2 * doc['popularity'].value) "

But what I do not understand is what is done with sqrt(1.2 * doc['popularity'].value) ? Is it multiplied by the original score of each hit to create a new score? Is it added? Can I change whether it is multiplied or added?

Is that what is defined in function_score["boost_mode"]?

Answer 1

yeah you are in the right direction. Two properties control the overall combination of individual scores and the score for the function score and naturally evaluated score. They are

• score_mode - This variable control how the computed scores are combined:

• boost_mode - This variable control how query score and computed score are combined

Reference

Take a look at the following query

{
    "query": {
        "function_score": {
            "query": {
                "match_all": {}
            },
            "functions": [{
                "field_value_factor": {
                    "field": "popularity",
                    "factor": 1.2,
                    "modifier": "sqrt",
                    "missing": 1
                }
            }, {
                "linear": {
                    "distance": {
                        "origin": "0",
                        "scale": "0.4"
                    }
                }
            }, {
                "gauss": {
                    "price": {
                        "origin": "0",
                        "scale": ".08"
                    }
                }
            }],
            "score_mode": "multiply",
            "boost_mode": "sum"
        }
    }
}

Since score_mode is multiply, as you can see there are three functions in my function score query, so this will multiply the score of each fucntion

function_score = score_linear * score_gauss * score_field_value_factor

Again - boost_mode is sum, so my final score will the summations of overall score evaluated by function score and the query score.

document_score = function_score + query_score.

Thanks