映射与统计

# 映射与统计 当我们在进行搜索的事情，我们会发现有一些奇怪的事情。比如有一些内容似乎是被打破了：在我们的索引中有12条推文，中有一个包含了`2014-09-15`这个日期，但是看看下面的查询结果中的总数量： ~~~ GET /_search?q=2014 # 12 results GET /_search?q=2014-09-15 # 12 results ! GET /_search?q=date:2014-09-15 # 1 result GET /_search?q=date:2014 # 0 results ! ~~~ 为什么我们使用字段`_all`搜索全年就会返回所有推文，而使用字段`date`搜索年份却没有结果呢？为什么使用两者所得到的结果是不同的？ 推测大概是因为我们的数据在`_all`和`date`在索引时没有被相同处理。我们来看看Elasticsearch是如何处理我们的文档结构的。我们可以对`gb`的`tweet`使用_mapping_请求： ~~~ GET /gb/_mapping/tweet ~~~ 我们得到： ~~~ { "gb": { "mappings": { "tweet": { "properties": { "date": { "type": "date", "format": "dateOptionalTime" }, "name": { "type": "string" }, "tweet": { "type": "string" }, "user_id": { "type": "long" } } } } } } ~~~ Elasticsearch会根据系统自动判断字段类型并生成一个映射。返回结果告诉我们`date`字段被识别成了`date`类型。`_all`没有出现是因为他是默认字段，但是我们知道字段`_all`实际上是`string`类型的。 所以类型为`date`的字段和类型为`string`的字段的索引方式是不同的。 So fields of type `date` and fields of type `string` are indexed differently,and can thus be searched differently. That's not entirely surprising.You might expect that each of the core data types -- strings, numbers, booleansand dates -- might be indexed slightly differently. And this is true:there are slight differences. But by far the biggest difference is actually between fields that represent_exact values_ (which can include `string` fields) and fields thatrepresent _full text_. This distinction is really important -- it's the thingthat separates a search engine from all other databases.