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This information applies to version 2.x of Elasticsearch. For the most up to date information, see the current version of the Elasticsearch Reference.
Elasticsearch: The Definitive Guide [2.x] » Aggregations » Sorting Multivalue Buckets » Intrinsic Sorts
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Intrinsic Sorts

These sort modes are intrinsic to the bucket: they operate on data that bucket generates, such as doc_count. They share the same syntax but differ slightly depending on the bucket being used.

Let’s perform a terms aggregation but sort by doc_count, in ascending order:

GET /cars/transactions/_search
{
    "size" : 0,
    "aggs" : {
        "colors" : {
            "terms" : {
              "field" : "color",
              "order": {
                "_count" : "asc" 
              }
            }
        }
    }
}

Using the _count keyword, we can sort by doc_count, in ascending order.

We introduce an order object into the aggregation, which allows us to sort on one of several values:

_count
Sort by document count. Works with terms, histogram, date_histogram.
_term
Sort by the string value of a term alphabetically. Works only with terms.
_key
Sort by the numeric value of each bucket’s key (conceptually similar to _term). Works only with histogram and date_histogram.
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