Elasticsearch function_score不在解释结果中

Question

我试图让function_score正常工作，但它似乎被忽略了。这是我的疑问：

{
  "explain" : true,
  "query": {
    "function_score": {
      "functions": [
        {
          "script_score": {
            "script": "_score * 999 * doc['total_digital_items_sold'].value"
          }
        }
      ],
      "query": {
        "filtered": {
          "query": {
             "query_string": {
               "query": "photo",
                 "fields": [ "title^3" ],
                 "use_dis_max": true,
                 "analyzer": "snowball"
               }
             },
             "filter": { "and": [ { } ] }
          }
        }
      }
    }
  }
}

它似乎不会影响结果，也不会出现在解释中。关于我做错了什么想法？