The purpose of this is to aggregate parent documents with the join data type field:

• Example: The purpose is to obtain a count of symbols that have dividends for the ETF in the cf_etf_dividend_join index:

"aggs": {"dividend":{ "parent": {"type": "dividend" },
    "aggs": {"count":{"value_count":{"field":"symbol"}}}}}

• Resulting buckets: Since we have only one symbol ACWF, the count value is 1:

 "aggregations": { "dividend": { "doc_count": 1, "count": { "value": 1 } } }