Global ordinals is a data-structure on top of doc values, that maintains an incremental numbering for each unique term in a lexicographic order. Each term has a unique number and the number of term A is lower than the number of term B. Global ordinals are only supported with keyword and text fields. In keyword fields, they are available by default but text fields can only use them when fielddata, with all of its associated baggage, is enabled.

Doc values (and fielddata) also have ordinals, which is a unique numbering for all terms in a particular segment and field. Global ordinals just build on top of this, by providing a mapping between the segment ordinals and the global ordinals, the latter being unique across the entire shard. Given that global ordinals for a specific field are tied to all the segments of a shard, they need to be entirely rebuilt whenever a once new segment becomes visible.

Global ordinals are used for features that use segment ordinals, such as the terms aggregation, to improve the execution time. A terms aggregation relies purely on global ordinals to perform the aggregation at the shard level, then converts global ordinals to the real term only for the final reduce phase, which combines results from different shards.

The loading time of global ordinals depends on the number of terms in a field, but in general it is low, since it source field data has already been loaded. The memory overhead of global ordinals is a small because it is very efficiently compressed.

By default, global ordinals are loaded at search-time, which is the right trade-off if you are optimizing for indexing speed. However, if you are more interested in search speed, it could be interesting to set eager_global_ordinals: true on fields that you plan to use in terms aggregations:

PUT my_index/_mapping
{
  "properties": {
    "tags": {
      "type": "keyword",
      "eager_global_ordinals": true
    }
  }
}

This will shift the cost from search-time to refresh-time. Elasticsearch will make sure that global ordinals are built before publishing updates to the content of the index.

If you ever decide that you do not need to run terms aggregations on this field anymore, then you can disable eager loading of global ordinals at any time:

PUT my_index/_mapping
{
  "properties": {
    "tags": {
      "type": "keyword",
      "eager_global_ordinals": false
    }
  }
}