ElasticSearch 全文搜索

对文档执行全文检索，包括单个或多个单词或词组查询，返回匹配条件的搜索结果。
ElasticSearch 是基于Apache Lucene的搜索引擎，一个开源、免费信息检索软件库。基于HTTP协议web接口和无模式json文档方式提供分布式、全文检索引擎。

本文探究下 ElasticSearch REST API 并演示基于HTTP 请求的基本查询操作。

安装环境

安装ElasticSearch，请参考官方安装指南。

RESTfull API运行端口为9200，让我们使用下面curl命令测试程序是否运行正确：

curl -XGET 'http://localhost:9200/'

如果你观察到下面响应，表明实例已经启动成功：

name "USER-20170915LA"
cluster_name    "elasticsearch"
cluster_uuid    "7k8ij9iPT1uAkVqJxanAYQ"
version
number  "7.1.1"
build_flavor    "oss"
build_type  "zip"
build_hash  "7a013de"
build_date  "2019-05-23T14:04:00.380842Z"
build_snapshot  false
lucene_version  "8.0.0"
minimum_wire_compatibility_version  "6.8.0"
minimum_index_compatibility_version "6.0.0-beta1"
tagline "You Know, for Search"

安装Elasticsearch Head
为了执行命令，我们可以安装Elasticsearch Head，也有相应chrome的插件，从应用商店中搜索安装。先启动elasticSearch，点击Head插件：

索引文档

ElasticSearch是面向文档NoSql应用，主要用于存储和索引文档。索引创建或更新文档，有了索引，即可搜索、排序、过滤完整的文档————不仅是行列类型的数据。这是一种完全不同的数据思考方式，也是ElasticSearch能够执行复杂全文搜索的原因之一。

文档以JSON对象方式表示,因为其简单、简洁其易读。JSON序列化被大多数编程语言支持，集合成为NoSql软件的标准格式.

下面我们使用一些随机文本执行全文检索：

{"title": "He went","random_text": "He went such dare good fact. The small own seven saved man age."
}{"title": "He oppose","random_text": "He oppose at thrown desire of no. \Announcing impression unaffected day his are unreserved indulgence."
}{"title": "Repulsive questions","random_text": "Repulsive questions contented him few extensive supported."
}{"title": "Old education","random_text": "Old education him departure any arranging one prevailed."
}

索引文档之前，我们需要决定其存储在哪。ElasticSearch可以包括多个索引，每个索引可以多文档。我们打算使用下列schema：

text：索引名称
article：类型名称
id：文本实体的唯一ID

可以使用下面命令增加索引：

put http://localhost:9200/text/artice/1/
{"title": "He went","random_text": "He went such dare good fact. The small own seven saved man age."
}

这里使用id=1，其他文本实体也可以使用相同的命令进行增加，只是id是递增的。

当然我们也可以在插入索引之前，手动建立Mapping，明确哪些字段需要全文检索，哪些字段仅需要严格匹配。示例代码如下：

PUT /text
{ "mappings": { "artice":{"properties": { "title": { "type" : "text" },"desc": { "type" : "text" }}}}
}

检索文档

前面增加了4个文档，我们现在可以检索有多少个文档，使用下面命令：

GET http://localhost:9200/text/_count/
{"query": {"match_all": {}}
}

返回结果：

{
"count": 4,
"_shards": {
"total": 1,
"successful": 1,
"skipped": 0,
"failed": 0
}
}

与我们插入的文档数量相符，下面查询特定文档：

http://localhost:9200/text/artice/1/

查询结果如下：

{"_index": "text","_type": "artice","_id": "1","_version": 1,"_seq_no": 0,"_primary_term": 1,"found": true,"_source": {"title": "He went","random_text": "He went such dare good fact. The small own seven saved man age."}
}

返回结果是我们之前增加ID为1的文档。

查询文档

现在测试全文检索，使用下面命令：

GET 'localhost:9200/text/article/_search
{"query": {"match": {"random_text": "him departure"}}
}

返回结果如下：

{"took": 32,"timed_out": false,"_shards": {"total": 5,"successful": 5,"failed": 0},"hits": {"total": 2,"max_score": 1.4513469,"hits": [{"_index": "text","_type": "article","_id": "4","_score": 1.4513469,"_source": {"title": "Old education","random_text": "Old education him departure any arranging one prevailed."}},{"_index": "text","_type": "article","_id": "3","_score": 0.28582606,"_source": {"title": "Repulsive questions","random_text": "Repulsive questions contented him few extensive supported."}}]}
}

我们查询 “him departure”，获得两个不同score的查询结果。第一条结果很明显，因为完全包括查询文本，其得分为 1.4513469.

第二条结果是因为目标文档包括单次“him”，得分为0.28582606。

缺省情况下ElasticSearch 根据相关性得分对查询结果进行排序，即每个文档匹配程度。注意，第二条结果得分比第一条低，表示相关性低。

模糊(Fuzzy)查询

模糊查询处理两个“模糊”相似的单词，就好像它们是同一个单词一样。首先，我们需要定义什么是模糊。
Elasticsearch支持最大编辑距离，使用模糊度参数指定为2。模糊度参数可设置为AUTO，编辑距离可以为:

0 表示一个或两个字符的字符串
1 表示三个、四个或五个字符的字符串
2 表示多于五个字符的字符串

如果使用编辑距离为2，返回结果似乎不相关。为了使返回结果更好、性能更好，使用编辑距离为1。距离指的是Levenshtein距离，这是一个字符串度量，用于测量两个序列之间的差异。下面执行模糊搜索：

GET localhost:9200/text/article/_search
{ "query": { "match": { "random_text": {"query": "him departure","fuzziness": "2"}} }
}

返回结果：

{"took": 88,"timed_out": false,"_shards": {"total": 5,"successful": 5,"failed": 0},"hits": {"total": 4,"max_score": 1.5834423,"hits": [{"_index": "text","_type": "article","_id": "4","_score": 1.4513469,"_source": {"title": "Old education","random_text": "Old education him departure any arranging one prevailed."}},{"_index": "text","_type": "article","_id": "2","_score": 0.41093433,"_source": {"title": "He oppose","random_text":"He oppose at thrown desire of no. \ Announcing impression unaffected day his are unreserved indulgence."}},{"_index": "text","_type": "article","_id": "3","_score": 0.2876821,"_source": {"title": "Repulsive questions","random_text": "Repulsive questions contented him few extensive supported."}},{"_index": "text","_type": "article","_id": "1","_score": 0.0,"_source": {"title": "He went","random_text": "He went such dare good fact. The small own seven saved man age."}}]}
}

我们看到模糊查询返回结果更多。使用模糊查询需要小心，因为可能返回根本不相干的结果。

总结

本文我们主要解释了索引文档，使用ElasticSearch Rest Api执行全文检索查询文档。