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In a few previous PR's we restructured the ES|QL docs to make it possible to generate them dynamically. This PR just moves a few files around to make the query languages docs easier to work with, and a little more organized like the ES|QL docs. A bit part of this was setting up redirects to the new locations, so other repo's could correctly link to the elasticsearch docs.
29 lines
1.6 KiB
Markdown
29 lines
1.6 KiB
Markdown
---
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mapped_pages:
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- https://www.elastic.co/guide/en/elasticsearch/reference/current/joining-queries.html
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---
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# Joining queries [joining-queries]
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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.
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[`nested` query](/reference/query-languages/query-dsl/query-dsl-nested-query.md)
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: Documents may contain fields of type [`nested`](/reference/elasticsearch/mapping-reference/nested.md). These fields are used to index arrays of objects, where each object can be queried (with the `nested` query) as an independent document.
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[`has_child`](/reference/query-languages/query-dsl/query-dsl-has-child-query.md) and [`has_parent`](/reference/query-languages/query-dsl/query-dsl-has-parent-query.md) queries
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: A [`join` field relationship](/reference/elasticsearch/mapping-reference/parent-join.md) 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.
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Also see the [terms-lookup mechanism](/reference/query-languages/query-dsl/query-dsl-terms-query.md#query-dsl-terms-lookup) in the `terms` query, which allows you to build a `terms` query from values contained in another document.
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## Notes [joining-queries-notes]
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### Allow expensive queries [_allow_expensive_queries_2]
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Joining queries will not be executed if [`search.allow_expensive_queries`](/reference/query-languages/querydsl.md#query-dsl-allow-expensive-queries) is set to false.
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