When they want to take a business critical app that is using Enterprise Oracle running on Exadata, and move it to community Postgres running on Amazon RDS, because "it will be in the Cloud, and it will save us money..." No, no it won't.

Won't they save on licences at least? Or won't postgre manage to cope with all the data?

@netikras RDS is not intended for Enterprise operations, and Exadata is blazing fast. In order to get even close to comparable performance on AWS, they will need an EC2 instance, and in the long run, no money will be saved. A DBMS is much more than it's cost. You truly get what you pay for.