A `has_child` query is a query type in Elasticsearch that is used to search for parent documents based on the content of their child documents. This query is used to find parent documents that have at least one child document matching a specified query.
When a `has_child` query is executed, Elasticsearch searches for parent documents that have at least one child document that matches the specified query criteria. The query searches only in the child documents and returns the parent documents.
Here’s an example of a `has_child` query in Elasticsearch:
GET /my_index/_search
{
"query": {
"has_child": {
"type": "comment",
"query": {
"match": {
"body": "elasticsearch"
}
},
"score_mode": "max"
}
}
}
In this example, we are searching the `my_index` index for parent documents that have at least one child document of type `comment` containing the term “elasticsearch” in the `body` field. The `score_mode` parameter is set to “max” to return the maximum score of the matching child documents.
The `has_child` query also supports a variety of other features, such as filtering the child documents using a `bool` query, specifying a minimum number of child documents that must match the query, and using nested fields as child documents.
The `has_child` query provides a powerful and flexible way to search for parent documents based on the content of their child documents in Elasticsearch. However, it’s important to note that `has_child` queries can be computationally expensive and may not be suitable for large datasets or high-traffic applications. Additionally, the performance of `has_child` queries can be impacted by the amount and complexity of the child documents. Therefore, it’s important to carefully consider the use case and performance implications before using `has_child` queries in Elasticsearch.