Good questions about reporting. In the Mendix Cloud, the database is Postgres and that isn’t something you can configure. Mendix does not open up direct access to Mendix Cloud databases, so implementing your own stored procedure on the Mendix DB itself is not possible. If you run Mendix on your own databases and systems, then you can take reporting action on that database as you would any other.
How then could one get compex data out of Mendix then? You can use OQL to create specific Datasets and populate Reports with that data. That might be the closest thing to what you are after. I suspect you may find it to be underpowered compared to the traditional mode. Some people elect to use OData exposed endpoints render data in a 3rd party reporting tool.