json results parsing

   Found the json API page here https://www.postgresql.org/docs/9.3/static/functions-json.html 

For some APIs, it returns set of results, for example, json_each(json) and json_object_keys(). 

Basically, I want to call these json API inside a plugin, and the return value for these APIs are Datum. I am wondering how to parse a set of json results from Datum in postgresql? Seems the helper functions I am aware of are only for simple types. Are there any examples to demonstrate the best practice of how to do that?

Thanks, Charles!