[[query-dsl-exists-filter]]
=== Exists Filter

Returns documents that have at least one non-`null` value in the original field:

[source,js]
--------------------------------------------------
{
    "constant_score" : {
        "filter" : {
            "exists" : { "field" : "user" }
        }
    }
}
--------------------------------------------------

For instance, these documents would all match the above filter:

[source,js]
--------------------------------------------------
{ "user": "jane" }
{ "user": "" } <1>
{ "user": "-" } <2>
{ "user": ["jane"] }
{ "user": ["jane", null ] } <3>
--------------------------------------------------
<1> An empty string is a non-`null` value.
<2> Even though the `standard` analyzer would emit zero tokens, the original field is non-`null`.
<3> At least one non-`null` value is required.

These documents would *not* match the above filter:

[source,js]
--------------------------------------------------
{ "user": null }
{ "user": [] } <1>
{ "user": [null] } <2>
{ "foo":  "bar" } <3>
--------------------------------------------------
<1> This field has no values.
<2> At least one non-`null` value is required.
<3> The `user` field is missing completely.

[float]
==== `null_value` mapping

If the field mapping includes the `null_value` setting (see <<mapping-core-types>>)
then explicit `null` values are replaced with the specified `null_value`.  For
instance, if the `user` field were mapped as follows:

[source,js]
--------------------------------------------------
  "user": {
    "type": "string",
    "null_value": "_null_"
  }
--------------------------------------------------

then explicit `null` values would be indexed as the string `_null_`, and the
following docs would match the `exists` filter:

[source,js]
--------------------------------------------------
{ "user": null }
{ "user": [null] }
--------------------------------------------------

However, these docs--without explicit `null` values--would still have
no values in the `user` field and thus would not match the `exists` filter:

[source,js]
--------------------------------------------------
{ "user": [] }
{ "foo": "bar" }
--------------------------------------------------


[float]
==== Caching

The result of the filter is always cached.
