十四、Elasticsearch的most fields策略

1、初步对比
（1）best-fields策略，主要是说将某一个field匹配尽可能多的关键词的doc，优先返回回来。

（2）most-fields策略，主要是说尽可能返回更多field匹配到某个关键词的doc，优先返回回来。

例如：
doc1：

{
  "title" : "china people",
  "content" : "i am a good person"
}

doc2：

{
  "title" : "china person",
  "content" : "i am a good people"
}

我现在要对title和content搜china person

best-fields策略：doc2优先被输出，因为他title字段两个关键词都匹配了
most-fields策略：doc1优先被输出，因为他title和content都包含关键词其中之一。

2、实战演练
（1）数据准备

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

（2）对sub_title字段进行搜索

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

结果：

{
  "took": 30,
  "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"
        }
      }
    ]
  }
}

为什么优先匹配出learned a lot of course？而且learned a lot of course的分数远大于learning more courses？

是因为我们在准备数据阶段，对sub_title使用的是English分词器，这种分词器会将单词还原为最近本的形态（stemmer）

learning --> learn
learned --> learn
courses --> course

sub_title：learning courses --> learn course

（3）这时候采取most-fields

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

结果：

{
  "took": 6,
  "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"
        }
      }
    ]
  }
}

大幅度提升了第二个的score。因为ub_title.std是standard分词的。

3、best-fields和most-fields区别

（1）best-fields是对多个field进行搜索，挑选某个field匹配度最高的那个分数，同时在多个query最高分相同的情况下，在一定程度上考虑其他query的分数，简单来说，你对多个field进行搜索，就想搜索到某一个field尽可能包含更多的关键字的数据。

优点：通过best_fields策略，以及综合考虑其他field，还有minimum_should_match支持，可以尽可能精准地将匹配的结果推送到最前面
缺点：除了那些精准匹配的结果，其他差不多大的结果，排序结果不是太均匀，没有什么区分度了

实际的例子：百度之类的搜索引擎，最匹配的到最前面，但是其他的就没什么区分度了

（2）most_fields，综合多个field一起进行搜索，尽可能多地让所有field的query参与到总分数的计算中来，此时就会是个大杂烩，出现类似best_fields案例最开始的那个结果，结果不一定精准，某一个document的一个field包含更多的关键字，但是因为其他document有更多field匹配到了，所以排在了前面；所以需要建立类似sub_title.std这样的field，尽可能让某一个field精准匹配query string，贡献更高的分数，将更精准匹配的数据排到前面

优点：将尽可能匹配更多field的结果推送到最前面，整个排序结果是比较均匀的
缺点：可能那些精准匹配的结果，无法推送到最前面

实际的例子：wiki，明显的most_fields策略，搜索结果比较均匀，但是的确要翻好几页才能找到最匹配的结果

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