How does a nested query work in Elasticsearch?

When a nested query is executed in Elasticsearch, the query searches for documents that contain at least one nested object that matches the specified query criteria. The nested query can be used to search for documents that contain a specific combination of nested objects, as well as to filter out documents that do not contain any nested objects.

Here’s an example of a nested query in Elasticsearch:

GET /my_index/_search
{
  "query": {
    "nested": {
      "path": "comments",
      "query": {
        "bool": {
          "must": [
            { "match": { "comments.author": "john" }},
            { "range": { "comments.timestamp": { "gte": "2022-01-01" }}} 
          ]
        }
      }
    }
  }
}

In this example, we are searching the `my_index` index for documents that contain at least one nested object in the `comments` field that matches the specified criteria. The nested query specifies the `comments` field as the `path` parameter and a `bool` query as the `query` parameter, which contains two must clauses that specify the author and timestamp of the nested object.

The nested query also supports a variety of other features, such as specifying multiple levels of nesting, using aggregations to group and aggregate nested objects, and using inner hits to return the matching nested objects.

The nested query provides a powerful and flexible way to search for nested objects in Elasticsearch, but it’s important to note that nested queries can be computationally expensive and may not be suitable for large datasets or high-traffic applications. Additionally, the nested query requires mapping the nested objects as nested fields in Elasticsearch, which can impact indexing and storage performance. Therefore, it’s important to carefully consider the use case and performance implications before using nested queries in Elasticsearch.