Function Score Queryedit

Note

Added in 0.90.4.

The function_score allows you to modify the score of documents that are retrieved by a query. This can be useful if, for example, a score function is computationally expensive and it is sufficient to compute the score on a filtered set of documents.

function_score provides the same functionality that Custom Boost Factor Query, Custom Score Query and Custom Filters Score Query provided but furthermore adds futher scoring functionality such as distance and recency scoring (see description below).

Using function scoreedit

To use function_score, the user has to define a query and one or several functions, that compute a new score for each document returned by the query.

function_score can be used with only one function like this:

"function_score": {
    "(query|filter)": {},
    "boost": "boost for the whole query",
    "FUNCTION": {},
    "boost_mode":"(multiply|replace|...)"
}

Furthermore, several functions can be combined. In this case one can optionally choose to apply the function only if a document matches a given filter:

"function_score": {
    "(query|filter)": {},
    "boost": "boost for the whole query",
    "functions": [
        {
            "filter": {},
            "FUNCTION": {}
        },
        {
            "FUNCTION": {}
        }
    ],
    "max_boost": number,
    "score_mode": "(multiply|max|...)",
    "boost_mode": "(multiply|replace|...)"
}

If no filter is given with a function this is equivalent to specifying "match_all": {}

First, each document is scored by the defined functons. The parameter score_mode specifies how the computed scores are combined:

multiply

scores are multiplied (default)

mult

scores are multiplied (default), deprecated [0.90.5, Renamed to multiply]

sum

scores are summed

avg

scores are averaged

first

the first function that has a matching filter is applied

max

maximum score is used

min

minimum score is used

The new score can be restricted to not exceed a certain limit by setting the max_boost parameter. The default for max_boost is FLT_MAX.

Finally, the newly computed score is combined with the score of the query. The parameter boost_mode defines how:

multiply
query score and function score is multiplied (default)
replace
only function score is used, the query score is ignored
sum
query score and function score are added
avg
average
max
max of query score and function score
min
min of query score and function score

Score functionsedit

The function_score query provides several types of score functions.

Script scoreedit

The script_score function allows you to wrap another query and customize the scoring of it optionally with a computation derived from other numeric field values in the doc using a script expression. Here is a simple sample:

"script_score" : {
    "script" : "_score * doc['my_numeric_field'].value"
}

On top of the different scripting field values and expression, the _score script parameter can be used to retrieve the score based on the wrapped query.

Scripts are cached for faster execution. If the script has parameters that it needs to take into account, it is preferable to reuse the same script, and provide parameters to it:

"script_score": {
    "lang": "lang",
    "params": {
        "param1": value1,
        "param2": value2
     },
    "script": "_score * doc['my_numeric_field'].value / pow(param1, param2)"
}

Note that unlike the Custom Score Query, the score of the query is multiplied with the result of the script scoring. If you wish to inhibit this, set "boost_mode": "replace"

Boost factoredit

The boost_factor score allows you to multiply the score by the provided boost_factor. This can sometimes be desired since boost value set on specific queries gets normalized, while for this score function it does not.

"boost_factor" : number

Randomedit

The random_score generates scores via a pseudo random number algorithm that is initialized with a seed.

"random_score": {
    "seed" : number
}

Decay functionsedit

Decay functions score a document with a function that decays depending on the distance of a numeric field value of the document from a user given origin. This is similar to a range query, but with smooth edges instead of boxes.

To use distance scoring on a query that has numerical fields, the user has to define an origin and a scale for each field. The origin is needed to define the “central point” from which the distance is calculated, and the scale to define the rate of decay. The decay function is specified as

"DECAY_FUNCTION": {
    "FIELD_NAME": {
          "origin": "11, 12",
          "scale": "2km",
          "offset": "0km",
          "decay": 0.33
    }
}

where DECAY_FUNCTION can be "linear", "exp" and "gauss" (see below). The specified field must be a numeric field. In the above example, the field is a Geo Point Type and origin can be provided in geo format. scale and offset must be given with a unit in this case. If your field is a date field, you can set scale and offset as days, weeks, and so on. Example:

    "DECAY_FUNCTION": {
        "FIELD_NAME": {
              "origin": "2013-09-17",
              "scale": "10d",
              "offset": "5d",
              "decay" : 0.5
        }
    }

The format of the origin depends on the Date Format defined in your mapping. If you do not define the origin, the current time is used.

The offset and decay parameters are optional.

offset
If an offset is defined, the decay function will only compute a the decay function for documents with a distance greater that the defined offset. The default is 0.
decay
The decay parameter defines how documents are scored at the distance given at scale. If no decay is defined, documents at the distance scale will be scored 0.5.

In the first example, your documents might represents hotels and contain a geo location field. You want to compute a decay function depending on how far the hotel is from a given location. You might not immediately see what scale to choose for the gauss function, but you can say something like: "At a distance of 2km from the desired location, the score should be reduced by one third." The parameter "scale" will then be adjusted automatically to assure that the score function computes a score of 0.5 for hotels that are 2km away from the desired location.

In the second example, documents with a field value between 2013-09-12 and 2013-09-22 would get a weight of 1.0 and documents which are 15 days from that date a weight of 0.5.

The DECAY_FUNCTION determines the shape of the decay:

gauss

Normal decay, computed as:

images/Gaussian.png

exp

Exponential decay, computed as:

images/Exponential.png

linear

Linear decay, computed as:

images/Linear.png.

In contrast to the normal and exponential decay, this function actually sets the score to 0 if the field value exceeds twice the user given scale value.

Detailed exampleedit

Suppose you are searching for a hotel in a certain town. Your budget is limited. Also, you would like the hotel to be close to the town center, so the farther the hotel is from the desired location the less likely you are to check in.

You would like the query results that match your criterion (for example, "hotel, Nancy, non-smoker") to be scored with respect to distance to the town center and also the price.

Intuitively, you would like to define the town center as the origin and maybe you are willing to walk 2km to the town center from the hotel. In this case your origin for the location field is the town center and the scale is ~2km.

If your budget is low, you would probably prefer something cheap above something expensive. For the price field, the origin would be 0 Euros and the scale depends on how much you are willing to pay, for example 20 Euros.

In this example, the fields might be called "price" for the price of the hotel and "location" for the coordinates of this hotel.

The function for price in this case would be

"DECAY_FUNCTION": {
    "price": {
          "origin": "0",
          "scale": "20"
    }
}

and for location:

"DECAY_FUNCTION": {
    "location": {
          "origin": "11, 12",
          "scale": "2km"
    }
}

where DECAY_FUNCTION can be "linear", "exp" and "gauss".

Suppose you want to multiply these two functions on the original score, the request would look like this:

curl 'localhost:9200/hotels/_search/' -d '{
"query": {
    "function_score": {
        "functions": [
            {
                "DECAY_FUNCTION": {
                    "price": {
                        "origin": "0",
                        "scale": "20"
                    }
                }
            },
            {
                "DECAY_FUNCTION": {
                    "location": {
                        "origin": "11, 12",
                        "scale": "2km"
                    }
                }
            }
        ],
        "query": {
            "match": {
                "properties": "balcony"
            }
        },
        "score_mode": "multiply"
    }
}
}'

Next, we show how the computed score looks like for each of the three possible decay functions.

Normal decay, keyword gaussedit

When choosing gauss as the decay function in the above example, the contour and surface plot of the multiplier looks like this:

https://f.cloud.github.com/assets/4320215/768157/cd0e18a6-e898-11e2-9b3c-f0145078bd6f.png
https://f.cloud.github.com/assets/4320215/768160/ec43c928-e898-11e2-8e0d-f3c4519dbd89.png

Suppose your original search results matches three hotels :

  • "Backback Nap"
  • "Drink n Drive"
  • "BnB Bellevue".

"Drink n Drive" is pretty far from your defined location (nearly 2 km) and is not too cheap (about 13 Euros) so it gets a low factor a factor of 0.56. "BnB Bellevue" and "Backback Nap" are both pretty close to the defined location but "BnB Bellevue" is cheaper, so it gets a multiplier of 0.86 whereas "Backpack Nap" gets a value of 0.66.

Exponential decay, keyword expedit

When choosing exp as the decay function in the above example, the contour and surface plot of the multiplier looks like this:

https://f.cloud.github.com/assets/4320215/768161/082975c0-e899-11e2-86f7-174c3a729d64.png
https://f.cloud.github.com/assets/4320215/768162/0b606884-e899-11e2-907b-aefc77eefef6.png

Linear' decay, keyword linearedit

When choosing linear as the decay function in the above example, the contour and surface plot of the multiplier looks like this:

https://f.cloud.github.com/assets/4320215/768164/1775b0ca-e899-11e2-9f4a-776b406305c6.png
https://f.cloud.github.com/assets/4320215/768165/19d8b1aa-e899-11e2-91bc-6b0553e8d722.png

Supported fields for decay functionsedit

Only single valued numeric fields, including time and geo locations, are supported.

What if a field is missing?edit

If the numeric field is missing in the document, the function will return 1.

Relation to custom_boost, custom_score and custom_filters_scoreedit

The Custom Boost Factor Query

"custom_boost_factor": {
    "boost_factor": 5.2,
    "query": {...}
}

becomes

"function_score": {
    "boost_factor": 5.2,
    "query": {...}
}

The Custom Score Query

"custom_score": {
    "params": {
        "param1": 2,
        "param2": 3.1
    },
    "query": {...},
    "script": "_score * doc['my_numeric_field'].value / pow(param1, param2)"
}

becomes

"function_score": {
    "boost_mode": "replace",
    "query": {...},
    "script_score": {
        "params": {
            "param1": 2,
            "param2": 3.1
        },
        "script": "_score * doc['my_numeric_field'].value / pow(param1, param2)"
    }
}

and the Custom Filters Score Query

"custom_filters_score": {
    "filters": [
        {
            "boost_factor": "3",
            "filter": {...}
        },
        {
            "filter": {…},
            "script": "_score * doc['my_numeric_field'].value / pow(param1, param2)"
        }
    ],
    "params": {
        "param1": 2,
        "param2": 3.1
    },
    "query": {...},
    "score_mode": "first"
}

becomes:

"function_score": {
    "functions": [
        {
            "boost_factor": "3",
            "filter": {...}
        },
        {
            "filter": {...},
            "script_score": {
                "params": {
                    "param1": 2,
                    "param2": 3.1
                },
                "script": "_score * doc['my_numeric_field'].value / pow(param1, param2)"
            }
        }
    ],
    "query": {...},
    "score_mode": "first"
}