Unable to aggregate on _index

[berggren]

When using the aggregations module I am unable to get aggregated doc counts per index.

The following query DO NOT return the aggregated result per index:

{
  "query": {
    "match_all": {}
  },
  "size": 0,
  "aggs": {
    "type": {
      "terms": {
        "field": "_index"
      }
    }
  }
}

When I use Facets instead of aggs, I get the desired result:

{
  "query": {
    "match_all": {}
  },
  "size": 0,
  "facets": {
    "type": {
      "terms": {
        "field": "_index"
      }
    }
  }
}

Is this a known issue?

[clintongormley]

/cc @jpountz

Looks like the _index field is not indexed. Probably facets has a workaround for it?

[jpountz]

@clintongormley Facets indeed have special handling for this field.

I think we can have a cleaner fix for this, eg. by making field mappers responsible for producing field data, so that a field which is neither indexed nor doc-valued but knows what its field data looks like could return a non-empty instance. I need to think more about this...