OpenSearch/docs/reference/query-dsl/joining-queries.asciidoc

[[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.
Docs: Reorganised the Query DSL docs into families and explaing query vs filter context 2015-06-03 19:59:22 -04:00			`[[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>>::
Fix grammar issues in some docs This commit fixes some grammar issues in various docs. Closes #20751 Closes #20752 Closes #20754 Closes #20755 2016-10-05 08:54:32 -04:00			Documents may contain fields of type <<nested,`nested`>>. These
Docs: Reorganised the Query DSL docs into families and explaing query vs filter context 2015-06-03 19:59:22 -04:00			`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::
Remove usage of multi-types from the docs and added a page explaining type removal (#25543) Closes #25401 2017-07-05 06:30:19 -04:00			A <<parent-join,`join` field relationship>> can exist between
			documents within a single index. The `has_child` query returns parent
Docs: Reorganised the Query DSL docs into families and explaing query vs filter context 2015-06-03 19:59:22 -04:00			`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[]`

Cleanup ParentFieldMapper: * Remove remaining 1.x bwc logic. * Stop storing stored fields and indexed terms. The _parent field's only purpose is to support joins between parent and child type and only storing doc values is sufficient. * In the mapping the parent field mapper is now known under '{parent}#{child}' key, because this is the field the parent/child join uses too. * Added new sub fetch phase to lookup that _parent field from doc values field if that is required (before this was fetched from stored _parent field) * Removed the ability to query directly on `_parent` in the query dsl. Instead the `{parent}#{child}` field should be used. Under the hood a doc values query is used instead of a term query, because only doc values fields are stored now. * Added a new `parent_id` query to easily query child documents with a specific parent id without having to know what join field to use * Also in aggregations `_parent` field can't be used any more and `{parent}#{child}` field name should be used instead to aggregate directly on the _parent join field. 2016-01-17 16:49:07 -05:00			`include::parent-id-query.asciidoc[]`

Add a cluster setting to disallow expensive queries (#51385) (#52279) Add a new cluster setting `search.allow_expensive_queries` which by default is `true`. If set to `false`, certain queries that have usually slow performance cannot be executed and an error message is returned. - Queries that need to do linear scans to identify matches: - Script queries - Queries that have a high up-front cost: - Fuzzy queries - Regexp queries - Prefix queries (without index_prefixes enabled - Wildcard queries - Range queries on text and keyword fields - Joining queries - HasParent queries - HasChild queries - ParentId queries - Nested queries - Queries on deprecated 6.x geo shapes (using PrefixTree implementation) - Queries that may have a high per-document cost: - Script score queries - Percolate queries Closes: #29050 (cherry picked from commit a8b39ed842c7770bd9275958c9f747502fd9a3ea) 2020-02-12 16:56:14 -05:00			`=== Notes`
			`==== Allow expensive queries`
			Joining queries will not be executed if <<query-dsl-allow-expensive-queries, `search.allow_expensive_queries`>>
			`is set to false.`