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[9.0] [DOCS] Update esql-lookup-join.md to mention index mode requirement (#126901) #126904

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Apr 16, 2025
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[9.0] [DOCS] Update esql-lookup-join.md to mention index mode requirement (#126901) #126904

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7 changes: 4 additions & 3 deletions docs/reference/query-languages/esql/esql-lookup-join.md
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
---
navigation_title: "Correlate data with LOOKUP JOIN"
mapped_pages:
- https://www.elastic.co/guide/en/elasticsearch/reference/current/esql-enrich-data.html
- https://www.elastic.co/guide/en/elasticsearch/reference/8.18/_lookup_join.html
---

# LOOKUP JOIN [esql-lookup-join-reference]
Expand Down Expand Up @@ -35,7 +35,7 @@ The `LOOKUP JOIN` command adds new columns to a table, with data from {{es}} ind
:::

`<lookup_index>`
: The name of the lookup index. This must be a specific index name - wildcards, aliases, and remote cluster references are not supported.
: The name of the lookup index. This must be a specific index name - wildcards, aliases, and remote cluster references are not supported. Indices used for lookups must be configured with the [`lookup` index mode](/reference/elasticsearch/index-settings/index-modules.md#index-mode-setting).

`<field_name>`
: The field to join on. This field must exist in both your current query results and in the lookup index. If the field contains multi-valued entries, those entries will not match anything (the added fields will contain `null` for those rows).
Expand Down Expand Up @@ -107,6 +107,7 @@ FROM employees

To use `LOOKUP JOIN`, the following requirements must be met:

* Indices used for lookups must be configured with the [`lookup` index mode](/reference/elasticsearch/index-settings/index-modules.md#index-mode-setting)
* **Compatible data types**: The join key and join field in the lookup index must have compatible data types. This means:
* The data types must either be identical or be internally represented as the same type in {{esql}}
* Numeric types follow these compatibility rules:
Expand All @@ -122,7 +123,7 @@ For a complete list of supported data types and their internal representations,

The following are the current limitations with `LOOKUP JOIN`

* Indices in [lookup](/reference/elasticsearch/index-settings/index-modules.md#index-mode-setting) mode are always single-sharded.
* Indices in [`lookup` mode](/reference/elasticsearch/index-settings/index-modules.md#index-mode-setting) are always single-sharded.
* Cross cluster search is unsupported initially. Both source and lookup indices must be local.
* Currently, only matching on equality is supported.
* `LOOKUP JOIN` can only use a single match field and a single index. Wildcards, aliases, datemath, and datastreams are not supported.
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