elasticsearch _source字段的一些说明

_source field

The _source field contains the original JSON document body that was passed at index time. The_source field itself is not indexed (and thus is not searchable), but it is stored so that it can be returned when executing fetch requests, like get or search.

Disabling the _source field

Though very handy to have around, the source field does incur storage overhead within the index. For this reason, it can be disabled as follows:

PUT tweets
{
  "mappings": {
    "tweet": {
      "_source": {
        "enabled": false
      }
    }
  }
}

COPY AS CURLVIEW IN CONSOLE 

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.

If disk space is a concern, rather increase the compression level instead of disabling the _source.

The metrics use case

The metrics use case is distinct from other time-based or logging use cases in that there are many small documents which consist only of numbers, dates, or keywords. There are no updates, no highlighting requests, and the data ages quickly so there is no need to reindex. Search requests typically use simple queries to filter the dataset by date or tags, and the results are returned as aggregations.

In this case, disabling the _source field will save space and reduce I/O. It is also advisable to disable the _all field in the metrics case.

Including / Excluding fields from _source

An expert-only feature is the ability to prune the contents of the _source field after the document has been indexed, but before the _source field is stored.

Removing fields from the _source has similar downsides to disabling _source, especially the fact that you cannot reindex documents from one Elasticsearch index to another. Consider using source filtering instead.

The includes/excludes parameters (which also accept wildcards) can be used as follows:

PUT logs
{
  "mappings": {
    "event": {
      "_source": {
        "includes": [
          "*.count",
          "meta.*"
        ],
        "excludes": [
          "meta.description",
          "meta.other.*"
        ]
      }
    }
  }
}

PUT logs/event/1
{
  "requests": {
    "count": 10,
    "foo": "bar" 
  },
  "meta": {
    "name": "Some metric",
    "description": "Some metric description", 
    "other": {
      "foo": "one", 
      "baz": "two" 
    }
  }
}

GET logs/event/_search
{
  "query": {
    "match": {
      "meta.other.foo": "one" 
    }
  }
}

COPY AS CURLVIEW IN CONSOLE 

	These fields will be removed from the stored _source field.
	We can still search on this field, even though it is not in the stored _source.