I think what they are referring to is that Postgres and most other databases were built in the before time, the long long ago when every connection was a process and you limited concurrency of connections in the configuration file. If you have a 1000 concurrent calls on lambda you aren't going to be able to have them all talking to the same database at the same time. You'll run out of connections and the application will crash. Same reason you see this happen to PHP web applications when HN or Slashdot is pointed at them and they say they can't connect to the database. They have hit the concurrency limit. Connection pooling solves this problem but currently requires another layer between the application and the database.