Allows to return the field data representation of a field for each hit, for example:

{
    "query" : {
        ...
    },
    "fielddata_fields" : ["test1", "test2"]
}

Field data fields can work on fields that are not stored.

It’s important to understand that using the fielddata_fields parameter will cause the terms for that field to be loaded to memory (cached), which will result in more memory consumption.