I wrote a Postgres Database Source in Python. But I cannot change it to be unbounded streaming. Can you write a Database Source in Scala og Java and use from PyFlink? I have not found any examples of mixed PyFlink and Java/Scala Flink projects.

You could build it into a JAR and then you could access it using the following way: https://nightlies.apache.org/flink/flink-docs-release-1.16/docs/dev/python/dependency_management/#jar-dependencies

Thanks I am now following this example for my work: https://flink.apache.org/2021/09/07/connector-table-sql-api-part1.html