The Java Database Connection (JDBC) importer allows to fetch data from JDBC sources for indexing into Elasticsearch.
The JDBC importer was designed for tabular data. If you have tables with many joins, the JDBC importer is limited in the way to reconstruct deeply nested objects to JSON and process object semantics like object identity.
Though it would be possible to extend the JDBC importer with a mapping feature where all the object properties could be specified, the current solution is focused on rather simple tabular data streams.
https://github.com/jprante/elasticsearch-jdbc