$json_arr = array(
            "terms"=>array(
            "field"=>"p_make",
            "limit"=>$top_n,
            "facet"=>array(
                "avg"=>'avg(cost_num)',
                "model"=>array(
                "terms"=>array(
                        "field"=>"p_model",
                    "limit"=>$top_n,
                    "mincount"=>2,
                    "facet"=>array(
                        "avg"=>'avg(cost_num)',
                    ),
                ),
                ),
            ),
            ),
        );

        $bbsSearch->addJsonFacet("make", $json_arr);