How to get the total count of unique terms on aggregations with the size set?

Question

When using the Terms Aggregation on an ElasticSearch query, the result will limit the buckets to the top 10 items or the value set on the size parameter. For example:

{
  "aggs" : {
    "cities" : {
      "terms" : { 
        "field" : "city",
        "size": 20
      }
    }
  }
}

This query would give me the top 20 buckets and their counts. How do I change this query to know the total count of unique "city" terms, so I can present something like "showing the top 20 cities of 73"?

Answer 1

The Cardinality Aggregation can be requested on the same query. So on the provided example, we would have:

{
  "aggs" : {
    "cities" : {
      "terms" : { 
        "field" : "city",
        "size": 20
      }
    },
    "unique_cities": {
      "cardinality": {
        "field": "city"
      }
    }
  }
}

And the "aggregations" response would have, besides the "cities" element (which contains the buckets), the "unique_cities" element with the cardinality:

"unique_cities": {
  "value": 73
}

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Credits to this issue on github: Return number of buckets for terms aggregation