Decay functions should allow to specify a value in case a field is missing

[FlorianWilhelm]

Describe the feature:

When using a decay function in a function_score the documentation says in the offical documentation: "If the numeric field is missing in the document, the function will return 1." In many cases this is not intended since documents missing the field are scored with the highest possible value.

Similar to field_value_factor decay functions should provide a missing parameter allowing to define the score in case the field is missing.

[clintongormley]

I think this makes sense. @brwe what do you think?

[brwe]

Last time we had the discussion we decided we will not do anything because there is a workaround: #7788 We can re evaluate this decision. Might make sense to make it consistent with field_value_factor. I have no strong opinion though.

[clintongormley]

+1 to consistency

[FlorianWilhelm]

@brwe Thank you, I haven't thought about that option actually but I guess I am not the only one and consistency is always better. I post your solution for completeness reasons here.

{
  "query": {
    "function_score": {
      "score_mode": "first",
      "functions": [
        {
          "filter": {
            "exists": {
              "field": "age"
            }
          },
          "gauss": {
            "age": {
              "origin": 22,
              "scale": 5,
              "decay": 0.5
            }
          }
        },
        {
          "script_score": {
            "script": "0"
          }
        }
      ]
    }
  }
}

[magicleo]

what if I want to use other functions and need "score_mode": "sum"?

[FlorianWilhelm]

@magicleo I would say that score_mode: first is only an optimization at that point. If you have sum you will just add 0. sometimes.

[kaka19ace]

At most usage cases, we disabled the script option for security reason, so we could not using decay function if the doc field not exists,

Using missing value is a good idea :)

[matthuhiggins]

kaka19ace - I'm having the same pain. Scripting is not enabled, so the suggested workaround is not available. Makes it tough to use the feature on coordinate fields.

[tuzz]

I wasn't able to use the workaround above because I have more than one function in function_score. Instead, I found another workaround which is to copy the nullable field to a new field in the index mapping to guarantee a value:

"mappings": {
  "name_of_type": {
    "field_that_might_be_null": {
      "type": "float",
      "copy_to": "field_that_definitely_wont_be_null"
    },
    "field_that_definitely_wont_be_null" {
      "type": "float",
      "null_value": 0
    }
  }
}

Depending on the type of decay, you may need to pick a default value that's far enough out of range to result in a value of 0. Hopefully that helps someone.