Elasticsearch uses _uid internally for identification because all documents land in the same Lucene indices. The type and ID separation is an abstraction that makes it easy to work with different structures by dividing them into types. _id is normally extracted from _uid because of that , but _type has to beindexed separately so it can easily filter documents by type when you search in a specific type.

Elasticsearch内部使用_uid进行标识，因为所有文档都位于相同的Lucene索引中。类型和ID分离是一种抽象，通过将不同的结构划分为类型，可以很容易地处理它们。因为这个原因，_id通常是从_uid中提取出来的，但是_type必须单独建立索引，以便在搜索特定类型的文档时能够根据类型轻松地过滤文档.