Myindex/_stats?filter_path=indices.**.query_cache got {}

[yc1024]

{
"query": {
    "bool":{
        "filter":{
           "bool":{
             "must":[
                   {"match_phrase" : {  "text" :"123" }},
					{"match_phrase" : {  "text" :"245" }},
					{"match_phrase" : {  "text" :"789" }}
                  ]
           }
        }
    }
}

}

I use the above DSL to query, and the results of three times are getting faster and faster.But， why myindex/_stats?filter_path=indices.**.query_cache got {}