[[joining-queries]] == 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. <<query-dsl-nested-query,`nested` query>>:: Documents may contain fields of type <<nested,`nested`>>. These fields are used to index arrays of objects, where each object can be queried (with the `nested` query) as an independent document. <<query-dsl-has-child-query,`has_child`>> and <<query-dsl-has-parent-query,`has_parent`>> queries:: A <<parent-join,`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 <<query-dsl-terms-lookup,terms-lookup mechanism>> in the `terms` query, which allows you to build a `terms` query from values contained in another document. include::nested-query.asciidoc[] include::has-child-query.asciidoc[] include::has-parent-query.asciidoc[] include::parent-id-query.asciidoc[] === Notes ==== Allow expensive queries Joining queries will not be executed if <<query-dsl-allow-expensive-queries, `search.allow_expensive_queries`>> is set to false.