﻿---
title: Using the murmur3 field
description: The murmur3 is typically used within a multi-field, so that both the original value and its hash are stored in the index: Such a mapping would allow to...
url: https://www.elastic.co/elastic/docs-builder/docs/2865/reference/elasticsearch/plugins/mapper-murmur3-usage
products:
  - Elasticsearch
---

# 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:
```json

{
  "mappings": {
    "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:
```json
# Example documents

{
  "my_field": "This is a document"
}


{
  "my_field": "This is another document"
}


{
  "aggs": {
    "my_field_cardinality": {
      "cardinality": {
        "field": "my_field.hash" <1>
      }
    }
  }
}
```

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.