[[mapper-murmur3]] === Mapper Murmur3 Plugin The mapper-murmur3 plugin provides the ability to compute hash of field values at index-time and store them in the index. This can sometimes be helpful when running cardinality aggregations on high-cardinality and large string fields. [[mapper-murmur3-install]] [float] ==== Installation This plugin can be installed using the plugin manager: [source,sh] ---------------------------------------------------------------- sudo bin/elasticsearch-plugin install mapper-murmur3 ---------------------------------------------------------------- The plugin must be installed on every node in the cluster, and each node must be restarted after installation. [[mapper-murmur3-remove]] [float] ==== Removal The plugin can be removed with the following command: [source,sh] ---------------------------------------------------------------- sudo bin/elasticsearch-plugin remove mapper-murmur3 ---------------------------------------------------------------- The node must be stopped before removing the plugin. [[mapper-murmur3-usage]] ==== 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: [source,js] -------------------------- PUT my_index { "mappings": { "my_type": { "properties": { "my_field": { "type": "keyword", "fields": { "hash": { "type": "murmur3" } } } } } } } -------------------------- // CONSOLE 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: [source,js] -------------------------- # 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" <1> } } } } -------------------------- // CONSOLE <1> 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.