_ignored field

The _ignored field indexes and stores the names of every field in a document that has been ignored when the document was indexed. This can, for example, be the case:

When the field was malformed and ignore_malformed was turned on.
When a keyword field’s value exceeds its optional ignore_above setting.
When index.mapping.total_fields.limit has been reached and index.mapping.total_fields.ignore_dynamic_beyond_limit is set to true.
The Elastic Stack includes Elastic's core products such as Elasticsearch, Kibana, Logstash, and Beats. Availability Planned We plan to add this functionality in a future Elastic Stack update. Subject to changes. Unless stated otherwise on the page, this functionality is available when your Elastic Stack is deployed on Elastic Cloud Hosted, Elastic Cloud Enterprise, Elastic Cloud on Kubernetes, and self-managed environments. This documentation corresponds to the latest patch available for each minor version. If you're not using the latest patch, check the release notes for changes. Elastic Cloud Serverless projects are autoscaled environments, fully managed by Elastic and available on Elastic Cloud. Availability Preview This functionality is in technical preview and is ready for evaluation. Use with caution in production; it is not recommended for mission-critical workloads. Elastic will work to fix any issues, but features in technical preview are not subject to the support SLA of official GA features. Specific Support terms apply. Serverless interfaces and procedures might differ from classic Elastic Stack deployments. When a field in a columnar index configured with on_failure: ignore receives a document that violates its multi_value: false or nullability: false constraint.
The Elastic Stack includes Elastic's core products such as Elasticsearch, Kibana, Logstash, and Beats. Availability Planned We plan to add this functionality in a future Elastic Stack update. Subject to changes. Unless stated otherwise on the page, this functionality is available when your Elastic Stack is deployed on Elastic Cloud Hosted, Elastic Cloud Enterprise, Elastic Cloud on Kubernetes, and self-managed environments. This documentation corresponds to the latest patch available for each minor version. If you're not using the latest patch, check the release notes for changes. Elastic Cloud Serverless projects are autoscaled environments, fully managed by Elastic and available on Elastic Cloud. Availability Preview This functionality is in technical preview and is ready for evaluation. Use with caution in production; it is not recommended for mission-critical workloads. Elastic will work to fix any issues, but features in technical preview are not subject to the support SLA of official GA features. Specific Support terms apply. Serverless interfaces and procedures might differ from classic Elastic Stack deployments. When a field in a columnar index was malformed and ignore_malformed was turned on. In columnar indices, ignore_above never adds a field to _ignored, because no value is dropped.

For more index setting details, refer to Mapping limit settings.

This field is searchable with term, terms and exists queries, and is returned as part of the search hits.

For instance the below query matches all documents that have one or more fields that got ignored:

GET _search
{
  "query": {
    "exists": {
      "field": "_ignored"
    }
  }
}

Similarly, the below query finds all documents whose @timestamp field was ignored at index time:

GET _search
{
  "query": {
    "term": {
      "_ignored": "@timestamp"
    }
  }
}

Since 8.15.0, the _ignored field supports aggregations as well. For example, the below query finds all fields that got ignored:

GET _search
{
  "aggs": {
    "ignored_fields": {
      "terms": {
         "field": "_ignored"
      }
    }
  }
}