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Nested aggregation

A special single bucket aggregation that enables aggregating nested documents.

For example, lets say we have an index of products, and each product holds the list of resellers - each having its own price for the product. The mapping could look like:

PUT /products
{
  "mappings": {
    "properties": {
      "resellers": { 
        "type": "nested",
        "properties": {
          "reseller": {
            "type": "keyword"
          },
          "price": {
            "type": "double"
          }
        }
      }
    }
  }
}

resellers is an array that holds nested documents.

The following request adds a product with two resellers:

PUT /products/_doc/0?refresh
{
  "name": "LED TV", 
  "resellers": [
    {
      "reseller": "companyA",
      "price": 350
    },
    {
      "reseller": "companyB",
      "price": 500
    }
  ]
}

We are using a dynamic mapping for the name attribute.

The following request returns the minimum price a product can be purchased for:

GET /products/_search?size=0
{
  "query": {
    "match": {
      "name": "led tv"
    }
  },
  "aggs": {
    "resellers": {
      "nested": {
        "path": "resellers"
      },
      "aggs": {
        "min_price": {
          "min": {
            "field": "resellers.price"
          }
        }
      }
    }
  }
}

As you can see above, the nested aggregation requires the path of the nested documents within the top level documents. Then one can define any type of aggregation over these nested documents.

Response:

{
  ...
  "aggregations": {
    "resellers": {
      "doc_count": 2,
      "min_price": {
        "value": 350.0
      }
    }
  }
}

You can use a filter sub-aggregation to return results for a specific reseller.

GET /products/_search?size=0
{
  "query": {
    "match": {
      "name": "led tv"
    }
  },
  "aggs": {
    "resellers": {
      "nested": {
        "path": "resellers"
      },
      "aggs": {
        "filter_reseller": {
          "filter": {
            "bool": {
              "filter": [
                {
                  "term": {
                    "resellers.reseller": "companyB"
                  }
                }
              ]
            }
          },
          "aggs": {
            "min_price": {
              "min": {
                "field": "resellers.price"
              }
            }
          }
        }
      }
    }
  }
}

The search returns:

{
  ...
  "aggregations": {
    "resellers": {
      "doc_count": 2,
      "filter_reseller": {
        "doc_count": 1,
        "min_price": {
          "value": 500.0
        }
      }
    }
  }
}

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https://www.elastic.co/guide/en/elasticsearch/reference/7.17/search-aggregations-bucket-nested-aggregation.html