How to add an additional field in elastic search ?I want to create an additional field to show time difference between two events in the new created field

Elastic Stack Kibana
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I want to create an additional field to show time difference between two events..

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Not easy. The EQL feature might help you to do that. See EQL search | Elasticsearch Guide [8.13] | Elastic. But I'm not sure on how to use that in Kibana.

I don't know (yet) how to do that with ES|QL if doable.

IMHO The best thing to do is to compute that at index time. The Logstash aggregate filter for example could help with that.