@King_Edward, you’re right that queries do run on the database, and not in Metabase itself. But we still have to generate the queries in Metabase and make sure the results come back with the right information that is needed to power features like drill-thru.
I just added a response to the GitHub issue you linked:
Ok, since people have been asking, to clarify, what we mean by saying we don’t support composite keys is that you can’t use them for features like Drill Thru and filtering by a column in a related table via composite foreign key and the like in the visual Query Builder. You can still run queries against these tables. You can still write any queries you want using SQL. This is only an issue with us not having a way to mode composite key relationships in the metadata used by the Query Builder. It’s something we will definitely address in the future.
Hopefully that clarifies things. Let me know if you have any more questions.