[[query-dsl-terms-query]]
=== Terms Query

Filters documents that have fields that match any of the provided terms
(*not analyzed*). For example:

[source,js]
--------------------------------------------------
GET /_search
{
    "query": {
        "terms" : { "user" : ["kimchy", "elasticsearch"]}
    }
}
--------------------------------------------------
// CONSOLE

NOTE: Highlighting `terms` queries is best-effort only, so terms of a `terms`
query might not be highlighted depending on the highlighter implementation that
is selected and on the number of terms in the `terms` query.

[float]
[[query-dsl-terms-lookup]]
===== Terms lookup mechanism

When it's needed to specify a `terms` filter with a lot of terms it can
be beneficial to fetch those term values from a document in an index. A
concrete example would be to filter tweets tweeted by your followers.
Potentially the amount of user ids specified in the terms filter can be
a lot. In this scenario it makes sense to use the terms filter's terms
lookup mechanism.

The terms lookup mechanism supports the following options:

[horizontal]
`index`::
    The index to fetch the term values from.

`type`::
    The type to fetch the term values from.

`id`::
    The id of the document to fetch the term values from.

`path`::
    The field specified as path to fetch the actual values for the
    `terms` filter.

`routing`::
    A custom routing value to be used when retrieving the
    external terms doc.

The values for the `terms` filter will be fetched from a field in a
document with the specified id in the specified type and index.
Internally a get request is executed to fetch the values from the
specified path. At the moment for this feature to work the `_source`
needs to be stored.

Also, consider using an index with a single shard and fully replicated
across all nodes if the "reference" terms data is not large. The lookup
terms filter will prefer to execute the get request on a local node if
possible, reducing the need for networking.

[WARNING]
Executing a Terms Query request with a lot of terms can be quite slow,
as each additional term demands extra processing and memory.
To safeguard against this, the maximum number of terms that can be used
in a Terms Query both directly or through lookup has been limited to `65536`.
This default maximum can be changed for a particular index with the index setting
 `index.max_terms_count`.

[float]
===== Terms lookup twitter example
At first we index the information for user with id 2, specifically, its
followers, then index a tweet from user with id 1. Finally we search on
all the tweets that match the followers of user 2.

[source,js]
--------------------------------------------------
PUT /users/_doc/2
{
    "followers" : ["1", "3"]
}

PUT /tweets/_doc/1
{
    "user" : "1"
}

GET /tweets/_search
{
    "query" : {
        "terms" : {
            "user" : {
                "index" : "users",
                "type" : "_doc",
                "id" : "2",
                "path" : "followers"
            }
        }
    }
}
--------------------------------------------------
// CONSOLE

The structure of the external terms document can also include an array of
inner objects, for example:

[source,js]
--------------------------------------------------
PUT /users/_doc/2
{
 "followers" : [
   {
     "id" : "1"
   },
   {
     "id" : "2"
   }
 ]
}
--------------------------------------------------
// CONSOLE

In which case, the lookup path will be `followers.id`.
