What you’d like to do currently not a native feature of postman. It does not have the ability to make connections to a database, and therefore, cannot perform what you are looking for.
What I might suggest, is if the backend can expose that composite key via the web API you’re testing, via a GET method, and then you can perform a a GET method first to check for the existence of the composite key before hitting the POST call. I am not sure if you have access or the ability to change it, but thats my recommendation as a work around.
Maybe Postman should make it a feature You could create a special table simply for integrations. URL, Method type etc. perhaps even Task schedual interval etc. You could also have a request and a response table. So basically you would just need three tables + a settings table and then your Postman exstension or whatever could run it Do you see where I´m getting at? ^^