[`percolator`]({{site.url}}{{site.baseurl}}/opensearch/supported-field-types/percolator/) | Specifies to treat this field as a query.
## Arrays
There is no dedicated array field type in OpenSearch. Instead, you can pass an array of values into any field. All values in the array must have the same field type.
```json
PUT testindex1/_doc/1
{
"number": 1
}
PUT testindex1/_doc/2
{
"number": [1, 2, 3]
}
```
## Multifields
Multifields are used to index the same field differently. Strings are often mapped as `text` for full-text queries and `keyword` for exact-value queries.
Multifields can be created using the `fields` parameter. For example, you can map a book `title` to be of type `text` and keep a `title.raw` subfield of type `keyword`.
```json
PUT books
{
"mappings" : {
"properties" : {
"title" : {
"type" : "text",
"fields" : {
"raw" : {
"type" : "keyword"
}
}
}
}
}
}
```
## Null value
Setting a field's value to `null`, an empty array or an array of `null` values makes this field equivalent to an empty field. Therefore, you cannot search for documents that have `null` in this field.
To make a field searchable for `null` values, you can specify its `null_value` parameter in the index's mappings. Then, all `null` values passed to this field will be replaced with the specified `null_value`.
The `null_value` parameter must be of the same type as the field. For example, if your field is a string, the `null_value` for this field must also be a string.
{: .note}
### Example
Create a mapping to replace `null` values in the `emergency_phone` field with the string "NONE":
```json
PUT testindex
{
"mappings": {
"properties": {
"name": {
"type": "keyword"
},
"emergency_phone": {
"type": "keyword",
"null_value": "NONE"
}
}
}
}
```
Index three documents into testindex. The `emergency_phone` fields of documents 1 and 3 contain `null`, while the `emergency_phone` field of document 2 has an empty array:
```json
PUT testindex/_doc/1
{
"name": "Akua Mansa",
"emergency_phone": null
}
```
```json
PUT testindex/_doc/2
{
"name": "Diego Ramirez",
"emergency_phone" : []
}
```
```json
PUT testindex/_doc/3
{
"name": "Jane Doe",
"emergency_phone": [null, null]
}
```
Search for people who do not have an emergency phone:
```json
GET testindex/_search
{
"query": {
"term": {
"emergency_phone": "NONE"
}
}
}
```
The response contains documents 1 and 3 but not document 2 because only explicit `null` values are replaced with the string "NONE":
```json
{
"took" : 1,
"timed_out" : false,
"_shards" : {
"total" : 1,
"successful" : 1,
"skipped" : 0,
"failed" : 0
},
"hits" : {
"total" : {
"value" : 2,
"relation" : "eq"
},
"max_score" : 0.18232156,
"hits" : [
{
"_index" : "testindex",
"_type" : "_doc",
"_id" : "1",
"_score" : 0.18232156,
"_source" : {
"name" : "Akua Mansa",
"emergency_phone" : null
}
},
{
"_index" : "testindex",
"_type" : "_doc",
"_id" : "3",
"_score" : 0.18232156,
"_source" : {
"name" : "Jane Doe",
"emergency_phone" : [
null,
null
]
}
}
]
}
}
```
The `_source` field still contains explicit `null` values because it is not affected by the `null_value`.
