null_value

A null value cannot be indexed or searched. When a field is set to null, (or an empty array or an array of null values) it is treated as though that field has no values.

The null_value parameter allows you to replace explicit null values with the specified value so that it can be indexed and searched. For instance:

 PUT my-index-000001 {
  "mappings": {
    "properties": {
      "status_code": {
        "type":       "keyword",
        "null_value": "NULL" 1
      }
    }
  }
}

PUT my-index-000001/_doc/1
{
  "status_code": null
}

PUT my-index-000001/_doc/2
{
  "status_code": [] 2
}

GET my-index-000001/_search
{
  "query": {
    "term": {
      "status_code": "NULL" 3
    }
  }
}

1. Replace explicit null values with the term NULL.
2. An empty array does not contain an explicit null, and so won’t be replaced with the null_value.
3. A query for NULL returns document 1, but not document 2.