Es 6.7 source include 单个字段的奇异现象

Note, this operation still means full reindex of the document, it just removes some network roundtrips and reduces chances of version conflicts between the get and the index. The _source field needs to be enabled for this feature to work.

参考：https://www.elastic.co/guide/e ... .html

Think before disabling the _source field

Users often disable the _source field without thinking about the consequences, and then live to regret it. If the _source field isn’t available then a number of features are not supported:

The update, update_by_query, and reindex APIs.
On the fly highlighting.
The ability to reindex from one Elasticsearch index to another, either to change mappings or analysis, or to upgrade an index to a new major version.
The ability to debug queries or aggregations by viewing the original document used at index time.
Potentially in the future, the ability to repair index corruption automatically.

参考：https://www.elastic.co/guide/e ... .html
 
按照官方文档：
如果_source不启用，update，update_by_query, reindex操作都是不能够使用的。
这是由于这些操作是先获取文档，将更新请求和原始文档的_source信息merge后，再重新index文档。
 
update是用来更新部分字段的，如果更新全部字段，可以使用index操作。_source不启用，不会影响index.