Elasticsearch aggregations enable you to get meta-information about your search results and answer questions like, "How many account holders are in Texas?" or "What’s the average balance of accounts in Tennessee?" You can search documents, filter hits, and use aggregations to analyze the results all in one request.

For example, the following request uses a terms aggregation to group all of the accounts in the bank index by state, and returns the ten states with the most accounts in descending order:

GET /bank/_search
{
  "size": 0,
  "aggs": {
    "group_by_state": {
      "terms": {
        "field": "state.keyword"
      }
    }
  }
}

The buckets in the response are the values of the state field. The doc_count shows the number of accounts in each state. For example, you can see that there are 27 accounts in ID (Idaho). Because the request set size=0, the response only contains the aggregation results.

{
  "took": 29,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "skipped" : 0,
    "failed": 0
  },
  "hits" : {
     "total" : {
        "value": 1000,
        "relation": "eq"
     },
    "max_score" : null,
    "hits" : [ ]
  },
  "aggregations" : {
    "group_by_state" : {
      "doc_count_error_upper_bound": 20,
      "sum_other_doc_count": 770,
      "buckets" : [ {
        "key" : "ID",
        "doc_count" : 27
      }, {
        "key" : "TX",
        "doc_count" : 27
      }, {
        "key" : "AL",
        "doc_count" : 25
      }, {
        "key" : "MD",
        "doc_count" : 25
      }, {
        "key" : "TN",
        "doc_count" : 23
      }, {
        "key" : "MA",
        "doc_count" : 21
      }, {
        "key" : "NC",
        "doc_count" : 21
      }, {
        "key" : "ND",
        "doc_count" : 21
      }, {
        "key" : "ME",
        "doc_count" : 20
      }, {
        "key" : "MO",
        "doc_count" : 20
      } ]
    }
  }
}

You can combine aggregations to build more complex summaries of your data. For example, the following request nests an avg aggregation within the previous group_by_state aggregation to calculate the average account balances for each state.

GET /bank/_search
{
  "size": 0,
  "aggs": {
    "group_by_state": {
      "terms": {
        "field": "state.keyword"
      },
      "aggs": {
        "average_balance": {
          "avg": {
            "field": "balance"
          }
        }
      }
    }
  }
}

Instead of sorting the results by count, you could sort using the result of the nested aggregation by specifying the order within the terms aggregation:

GET /bank/_search
{
  "size": 0,
  "aggs": {
    "group_by_state": {
      "terms": {
        "field": "state.keyword",
        "order": {
          "average_balance": "desc"
        }
      },
      "aggs": {
        "average_balance": {
          "avg": {
            "field": "balance"
          }
        }
      }
    }
  }
}

In addition to basic bucketing and metrics aggregations like these, Elasticsearch provides specialized aggregations for operating on multiple fields and analyzing particular types of data such as dates, IP addresses, and geo data. You can also feed the results of individual aggregations into pipeline aggregations for further analysis.

The core analysis capabilities provided by aggregations enable advanced features such as using machine learning to detect anomalies.