概述

继续跟中华石杉老师学习ES，第26篇

课程地址： https://www.roncoo.com/view/55

官方说明

https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-function-score-query.html

简单来说：自定义一个function_score函数，自己将某个field的值，跟es内置算出来的分数进行运算，然后由自己指定的field来进行分数的增强

例子

需求：看帖子的人越多，那么帖子的分数就越高

先给所有的帖子数据增加follower数量，将对帖子搜索得到的分数，跟follower_num进行运算，由follower_num在一定程度上增强帖子的分数
看帖子的人越多，那么帖子的分数就越高

POST /forum/article/_bulk
{ "update": { "_id": "1"} }
{ "doc" : {"follower_num" : 5} }
{ "update": { "_id": "2"} }
{ "doc" : {"follower_num" : 10} }
{ "update": { "_id": "3"} }
{ "doc" : {"follower_num" : 25} }
{ "update": { "_id": "4"} }
{ "doc" : {"follower_num" : 3} }
{ "update": { "_id": "5"} }
{ "doc" : {"follower_num" : 60} }

DSL

GET /forum/article/_search
{"query": {"function_score": {"query": {"multi_match": {"query": "java spark","fields": ["tile", "content"]}},"field_value_factor": {"field": "follower_num","modifier": "log1p","factor": 0.5},"boost_mode": "sum","max_boost": 5}}
}

如果只有field，那么会将每个doc的分数都乘以follower_num，如果有的doc follower是0，那么分数就会变为0，效果很不好。
因此一般会加个log1p函数，公式会变为，new_score = old_score * log(1 + number_of_votes)，这样出来的分数会比较合理。
再加个factor，可以进一步影响分数，new_score = old_score * log(1 + factor * number_of_votes)

boost_mode，可以决定分数与指定字段的值如何计算 : multiply，replace, sum，min，max，avg

max_boost，限制计算出来的分数不要超过max_boost指定的值

返回结果：

{"took": 87,"timed_out": false,"_shards": {"total": 1,"successful": 1,"skipped": 0,"failed": 0},"hits": {"total": 2,"max_score": 3.8050528,"hits": [{"_index": "forum","_type": "article","_id": "5","_score": 3.8050528,"_source": {"articleID": "DHJK-B-1395-#Ky5","userID": 3,"hidden": false,"postDate": "2019-05-01","tag": ["elasticsearch"],"tag_cnt": 1,"view_cnt": 10,"title": "this is spark blog","content": "spark is best big data solution based on scala ,an programming language similar to java spark","sub_title": "haha, hello world","author_first_name": "Tonny","author_last_name": "Peter Smith","new_author_last_name": "Peter Smith","new_author_first_name": "Tonny","follower_num": 60}},{"_index": "forum","_type": "article","_id": "2","_score": 1.7247463,"_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","author_first_name": "Smith","author_last_name": "Williams","new_author_last_name": "Williams","new_author_first_name": "Smith","follower_num": 10}}]}
}

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