Hi @SB699925, you are correct in that it will need to write back to a table which is why you need the SELECT 1 as test statement. The other thing I noticed in other people's comments is that you can't use the Sample button. You just have to run it.
It is possible that the functionality has changed as the posts around this subject are older and the product has changed quite a bit over time.
The other option is use something like PyODBC in a Python script to make the call. I spoke about that option in a post last year: https://matillioncommunity.discourse.group/t/can-i-call-sql-server-stored-procedures-from-matillion/1171
In your case, I might recommend a Python module called "psycopg2" as it's specifically built for Postgres and makes the configuration a bit more straightforward.
I don't have a Postgres instance I can connect to right now or I would test it myself. Hopefully this helps