Reindex performance

I want to move subset of data from one index to another (could be same cluster or different). What are the advantages of using a _reindex API instead of simply indexing the required data via logstash from the origin of data e.g RDBMS. My elasticsearch use case is of search.


if the data is already in Elasticsearch and you don't need complex transformations, the _reindex API might be a good choice. If the data is in an external system or you need complex transformations, Logstash might be a better choice.


okay, we have data already in Elasticsearch. If possible, could you please provide more details on how _reindex API will be better in this case?