In the prior examples, the metric was a direct child of the bucket. An average price was calculated for each term. It is possible to sort on deeper metrics, which are grandchildren or great-grandchildren of the bucket—​with some limitations.

You can define a path to a deeper, nested metric by using angle brackets (>), like so: my_bucket>another_bucket>metric.

The caveat is that each nested bucket in the path must be a single-value bucket. A filter bucket produces a single bucket: all documents that match the filtering criteria. Multivalue buckets (such as terms) generate many dynamic buckets, which makes it impossible to specify a deterministic path.

Currently, there are only three single-value buckets: filter, global, and reverse_nested. As a quick example, let’s build a histogram of car prices, but order the buckets by the variance in price of red and green (but not blue) cars in each price range:

GET /cars/transactions/_search
{
    "size" : 0,
    "aggs" : {
        "colors" : {
            "histogram" : {
              "field" : "price",
              "interval": 20000,
              "order": {
                "red_green_cars>stats.variance" : "asc" 
              }
            },
            "aggs": {
                "red_green_cars": {
                    "filter": { "terms": {"color": ["red", "green"]}}, 
                    "aggs": {
                        "stats": {"extended_stats": {"field" : "price"}} 
                    }
                }
            }
        }
    }
}

In this example, you can see that we are accessing a nested metric. The stats metric is a child of red_green_cars, which is in turn a child of colors. To sort on that metric, we define the path as red_green_cars>stats.variance. This is allowed because the filter bucket is a single-value bucket.