Using the murmur3 field

The murmur3 is typically used within a multi-field, so that both the original value and its hash are stored in the index:

PUT my_index
{
  "mappings": {
    "my_type": {
      "properties": {
        "my_field": {
          "type": "keyword",
          "fields": {
            "hash": {
              "type": "murmur3"
            }
          }
        }
      }
    }
  }
}

Such a mapping would allow to refer to my_field.hash in order to get hashes of the values of the my_field field. This is only useful in order to run cardinality aggregations:

# Example documents
PUT my_index/my_type/1
{
  "my_field": "This is a document"
}

PUT my_index/my_type/2
{
  "my_field": "This is another document"
}

GET my_index/_search
{
  "aggs": {
    "my_field_cardinality": {
      "cardinality": {
        "field": "my_field.hash" 
      }
    }
  }
}

Counting unique values on the my_field.hash field

Running a cardinality aggregation on the my_field field directly would yield the same result, however using my_field.hash instead might result in a speed-up if the field has a high-cardinality. On the other hand, it is discouraged to use the murmur3 field on numeric fields and string fields that are not almost unique as the use of a murmur3 field is unlikely to bring significant speed-ups, while increasing the amount of disk space required to store the index.