=== LOOKUP JOIN
++++
<titleabbrev>Correlate data with LOOKUP JOIN</titleabbrev>
++++

// hack because page didn't have explicit id originally we could link to using internal link syntax
[[esql-lookup-join-landing-page]]

[WARNING]
====
This functionality is in technical preview and may be
changed or removed in a future release. Elastic will work to fix any
issues, but features in technical preview are not subject to the support
SLA of official GA features.
====

The {esql} <<esql-lookup-join,LOOKUP join>>
processing command combines data from your {esql} query results
table with matching records from a specified lookup index. It adds
fields from the lookup index as new columns to your results table based
on matching values in the join field.

Teams often have data scattered across multiple indices – like logs,
IPs, user IDs, hosts, employees etc. Without a direct way to enrich or
correlate each event with reference data, root-cause analysis, security
checks, and operational insights become time-consuming.

For example, you can use `LOOKUP JOIN` to:

* Retrieve environment or ownership details for each host to correlate
your metrics data.
* Quickly see if any source IPs match known malicious addresses.
* Tag logs with the owning team or escalation info for faster triage and
incident response.

[discrete]
[[esql-compare-with-enrich]]
==== Compare with ENRICH

<<esql-lookup-join,LOOKUP join>> is similar to <<esql-enrich-data,ENRICH>>
in the fact that they both help you join data together. You should use
`LOOKUP JOIN` when:

* Your enrichment data changes frequently
* You want to avoid index-time processing
* You want SQL-like behavior, so that multiple matches result in multiple rows
* You need to match on any field in a lookup index
* You use document or field level security
* You want to restrict users to use only specific lookup indices
* You do not need to match using ranges or spatial relations

[discrete]
[[esql-how-lookup-join-works]]
==== How the command works

The `LOOKUP JOIN` command adds fields from the lookup index as new columns 
to your results table based on matching values in the join field.

[source,esql]
----
LOOKUP JOIN <lookup_index> ON <field_name>
----

The command requires two parameters:

[[esql-lookup-join-lookup-index]]
lookup_index::
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 <<index-mode-setting,`lookup` mode>>.

[[esql-lookup-join-field-name]]
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).

image::images/esql/esql-lookup-join.png[align="center"]

If you're familiar with SQL, `LOOKUP JOIN` has left-join behavior. This means that 
if no rows match in the lookup index, the incoming row is retained and `null` values are added. If many rows in the lookup index match, `LOOKUP JOIN` adds one row per match.

[discrete]
[[esql-lookup-join-example]]
==== Example

You can run this example for yourself to see how it works by setting up the indices and adding sample data. Otherwise, you just inspect the query and response.

[discrete]
[[esql-lookup-join-example-setup-sample-data]]
===== Sample data

.*Expand for setup instructions*
[%collapsible]
==============

**Set up indices**

First, let's create two indices with mappings: `threat_list` and `firewall_logs`.

[source,console]
----
PUT threat_list
{
  "settings": {
    "index.mode": "lookup" <1>
  },
  "mappings": {
    "properties": {
      "source.ip": { "type": "ip" },
      "threat_level": { "type": "keyword" },
      "threat_type": { "type": "keyword" },
      "last_updated": { "type": "date" }
    }
  }
}
----
<1> The lookup index must be set up using this mode

[source,console]
----
PUT firewall_logs
{
  "mappings": {
    "properties": {
      "timestamp": { "type": "date" },
      "source.ip": { "type": "ip" },
      "destination.ip": { "type": "ip" },
      "action": { "type": "keyword" },
      "bytes_transferred": { "type": "long" }
    }
  }
}
----

*Add sample data*

Next, let's add some sample data to both indices. The `threat_list` index contains known malicious IPs, while the `firewall_logs` index contains logs of network traffic.

[source,console]
----
POST threat_list/_bulk
{"index":{}}
{"source.ip":"203.0.113.5","threat_level":"high","threat_type":"C2_SERVER","last_updated":"2025-04-22"}
{"index":{}}
{"source.ip":"198.51.100.2","threat_level":"medium","threat_type":"SCANNER","last_updated":"2025-04-23"}
----

[source,console]
----
POST firewall_logs/_bulk
{"index":{}}
{"timestamp":"2025-04-23T10:00:01Z","source.ip":"192.0.2.1","destination.ip":"10.0.0.100","action":"allow","bytes_transferred":1024}
{"index":{}}
{"timestamp":"2025-04-23T10:00:05Z","source.ip":"203.0.113.5","destination.ip":"10.0.0.55","action":"allow","bytes_transferred":2048}
{"index":{}}
{"timestamp":"2025-04-23T10:00:08Z","source.ip":"198.51.100.2","destination.ip":"10.0.0.200","action":"block","bytes_transferred":0}
{"index":{}}
{"timestamp":"2025-04-23T10:00:15Z","source.ip":"203.0.113.5","destination.ip":"10.0.0.44","action":"allow","bytes_transferred":4096}
{"index":{}}
{"timestamp":"2025-04-23T10:00:30Z","source.ip":"192.0.2.1","destination.ip":"10.0.0.100","action":"allow","bytes_transferred":512}
----
==============

[discrete]
[[esql-lookup-join-example-query]]
===== Query the Data

[source,esql]
----
FROM firewall_logs <1>
| LOOKUP JOIN threat_list ON source.ip <2>
| WHERE threat_level IS NOT NULL <3>
| SORT timestamp <4>
| KEEP source.ip, action, threat_level, threat_type <5>
| LIMIT 10 <6>
----

<1> The source index
<2> The lookup index and join field
<3> Filter for rows with non-null threat levels
<4> LOOKUP JOIN does not guarantee output order, so you must explicitly sort
<5> Keep only relevant fields
<6> Limit the output to 10 rows

[discrete]
[[esql-lookup-join-example-response]]
===== Response

A successful query will output a table like this:

[cols="4*",options="header"]
|===
|source.ip    |action     |threat_type  |threat_level
|203.0.113.5  |allow      |C2_SERVER    |high
|198.51.100.2 |block      |SCANNER      |medium
|203.0.113.5  |allow      |C2_SERVER    |high
|===

In this example, you can see that the `source.ip` field from the `firewall_logs` index is matched with the `source.ip` field in the `threat_list` index, and the corresponding `threat_level` and `threat_type` fields are added to the output.

[discrete]
[[esql-lookup-join-additional-examples]]
===== Additional examples

Refer to the examples section of the <<esql-lookup-join,LOOKUP JOIN>> command reference for more examples.

[discrete]
[[esql-lookup-join-prereqs]]
==== Prerequisites

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

* Indices used for lookups must be configured with the <<index-mode-setting,`lookup` mode>> 
* *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:
*** `short` and `byte` are compatible with `integer` (all represented as
`int`)
*** `float`, `half_float`, and `scaled_float` are compatible
with `double` (all represented as `double`)
** For text fields: You can only use text fields as the join key on the
left-hand side of the join and only if they have a `.keyword` subfield

To obtain a join key with a compatible type, use a
<<esql-type-conversion-functions,conversion function>> if needed.

For a complete list of supported data types and their internal
representations, see the <<esql-supported-types,Supported Field Types documentation>>.

[discrete]
[[esql-lookup-join-limitations]]
==== Limitations

The following are the current limitations with `LOOKUP JOIN`

* Indices in <<index-mode-setting,`lookup`>> mode 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.
* The name of the match field in
`LOOKUP JOIN lu++_++idx ON match++_++field` must match an existing field
in the query. This may require renames or evals to achieve.
* The query will circuit break if there are too many matching documents
in the lookup index, or if the documents are too large. More precisely,
`LOOKUP JOIN` works in batches of, normally, about 10,000 rows; a large
amount of heap space is needed if the matching documents from the lookup
index for a batch are multiple megabytes or larger. This is roughly the
same as for `ENRICH`.
