You can absolutely use a Postgres not on Heroku with your Heroku app. The DATABASE_URL is the key part, there are customers on Heroku that use RDS or we've seen

You can absolutely use a Postgres not on Heroku with your Heroku app. The DATABASE_URL is the key part, there are customers on Heroku that use RDS or we've seen a few start to use Crunchy Bridge as well. As for VPC peering, that is going to be less likely. You'd have to at least be using Heroku's private spaces and even then I'm not sure it's a turnkey integration. You essentially would need a static/public connection for Heroku to be able to connect.