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Joining queries

Performing full SQL-style joins in a distributed system like Elasticsearch is prohibitively expensive. Instead, Elasticsearch offers two forms of join which are designed to scale horizontally.

nested query: Documents may contain fields of type nested. These fields are used to index arrays of objects, where each object can be queried (with the nested query) as an independent document.
has_child and has_parent queries: A join field relationship can exist between documents within a single index. The has_child query returns parent documents whose child documents match the specified query, while the has_parent query returns child documents whose parent document matches the specified query.

Also see the terms-lookup mechanism in the terms query, which allows you to build a terms query from values contained in another document.

Notes

Allow expensive queries

Joining queries will not be executed if search.allow_expensive_queries is set to false.

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https://www.elastic.co/guide/en/elasticsearch/reference/7.17/joining-queries.html