從best-fields換成most-fields策略
best-fields策略，主要是說將某一個field匹配盡可能多的關鍵詞的doc優先返回回來
most-fields策略，主要是說盡可能返回更多field匹配到某個關鍵詞的doc，優先返回回來

POST /forum/_mapping/article
{
  "properties": {
      "sub_title": { 
          "type":     "text",
          "analyzer": "english",
          "fields": {
              "std":   { 
                  "type":     "text",
                  "analyzer": "standard"
              }
          }
      }
  }
}

POST /forum/article/_bulk
{ "update": { "_id": "1"} }
{ "doc" : {"sub_title" : "learning more courses"} }
{ "update": { "_id": "2"} }
{ "doc" : {"sub_title" : "learned a lot of course"} }
{ "update": { "_id": "3"} }
{ "doc" : {"sub_title" : "we have a lot of fun"} }
{ "update": { "_id": "4"} }
{ "doc" : {"sub_title" : "both of them are good"} }
{ "update": { "_id": "5"} }
{ "doc" : {"sub_title" : "haha, hello world"} }

GET /forum/article/_search
{
  "query": {
    "match": {
      "sub_title": "learning courses"
    }
  }
}

{
  "took": 3,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "failed": 0
  },
  "hits": {
    "total": 2,
    "max_score": 1.219939,
    "hits": [
      {
        "_index": "forum",
        "_type": "article",
        "_id": "2",
        "_score": 1.219939,
        "_source": {
          "articleID": "KDKE-B-9947-#kL5",
          "userID": 1,
          "hidden": false,
          "postDate": "2017-01-02",
          "tag": [
            "java"
          ],
          "tag_cnt": 1,
          "view_cnt": 50,
          "title": "this is java blog",
          "content": "i think java is the best programming language",
          "sub_title": "learned a lot of course"
        }
      },
      {
        "_index": "forum",
        "_type": "article",
        "_id": "1",
        "_score": 0.5063205,
        "_source": {
          "articleID": "XHDK-A-1293-#fJ3",
          "userID": 1,
          "hidden": false,
          "postDate": "2017-01-01",
          "tag": [
            "java",
            "hadoop"
          ],
          "tag_cnt": 2,
          "view_cnt": 30,
          "title": "this is java and elasticsearch blog",
          "content": "i like to write best elasticsearch article",
          "sub_title": "learning more courses"
        }
      }
    ]
  }
}

sub_title用的是english analyzer，所以還原了單詞

為什么，因為如果我們用的是類似于english analyzer這種分詞器的話，就會將單詞還原為其最基本的形態，stemmer
learning --> learn
learned --> learn
courses --> course

sub_titile: learning coureses --> learn course

{ "doc" : {"sub_title" : "learned a lot of course"} }，就排在了{ "doc" : {"sub_title" : "learning more courses"} }的前面

GET /forum/article/_search
{
   "query": {
        "match": {
            "sub_title": "learning courses"
        }
    }
}

GET /forum/article/_search
{
   "query": {
        "multi_match": {
            "query":  "learning courses",
            "type":   "most_fields", 
            "fields": [ "sub_title", "sub_title.std" ]
        }
    }
}

{
  "took": 2,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "failed": 0
  },
  "hits": {
    "total": 2,
    "max_score": 1.219939,
    "hits": [
      {
        "_index": "forum",
        "_type": "article",
        "_id": "2",
        "_score": 1.219939,
        "_source": {
          "articleID": "KDKE-B-9947-#kL5",
          "userID": 1,
          "hidden": false,
          "postDate": "2017-01-02",
          "tag": [
            "java"
          ],
          "tag_cnt": 1,
          "view_cnt": 50,
          "title": "this is java blog",
          "content": "i think java is the best programming language",
          "sub_title": "learned a lot of course"
        }
      },
      {
        "_index": "forum",
        "_type": "article",
        "_id": "1",
        "_score": 1.012641,
        "_source": {
          "articleID": "XHDK-A-1293-#fJ3",
          "userID": 1,
          "hidden": false,
          "postDate": "2017-01-01",
          "tag": [
            "java",
            "hadoop"
          ],
          "tag_cnt": 2,
          "view_cnt": 30,
          "title": "this is java and elasticsearch blog",
          "content": "i like to write best elasticsearch article",
          "sub_title": "learning more courses"
        }
      }
    ]
  }
}

你問我，具體的分數怎么算出來的，很難說，因為這個東西很復雜， 還不只是TF/IDF算法。因為不同的query，不同的語法，都有不同的計算score的細節。

與best_fields的區別

（1）best_fields，是對多個field進行搜索，挑選某個field匹配度最高的那個分數，同時在多個query最高分相同的情況下，在一定程度上考慮其他query的分數。簡單來說，你對多個field進行搜索，

就想搜索到某一個field盡可能包含更多關鍵字的數據

優點：通過best_fields策略，以及綜合考慮其他field，還有minimum_should_match支持，可以盡可能精準地將匹配的結果推送到最前面
缺點：除了那些精準匹配的結果，其他差不多大的結果，排序結果不是太均勻，沒有什么區分度了

實際的例子：百度之類的搜索引擎，最匹配的到最前面，但是其他的就沒什么區分度了

（2）most_fields，綜合多個field一起進行搜索，盡可能多地讓所有field的query參與到總分數的計算中來，此時就會是個大雜燴，出現類似best_fields案例最開始的那個結果，結果不一定精準，某一

個document的一個field包含更多的關鍵字，但是因為其他document有更多field匹配到了，所以排在了前面；所以需要建立類似sub_title.std這樣的field，盡可能讓某一個field精準匹配query string

，貢獻更高的分數，將更精準匹配的數據排到前面

優點：將盡可能匹配更多field的結果推送到最前面，整個排序結果是比較均勻的
缺點：可能那些精準匹配的結果，無法推送到最前面

實際的例子：wiki，明顯的most_fields策略，搜索結果比較均勻，但是的確要翻好幾頁才能找到最匹配的結果