There's no reason completion notifications can't occur in an async API. It sounds like oracle calls polling "asynchronous", which is not particularly accurate. If the polling operations also involve network round trips to check for query completion, that is a tremendously stupid API.
Reason's simple: you have to support in your 11g server clients written in the dim and distant past, so you can't monkey with the protocol too much. You could do this as you suggest by subscribing for DCN events when your transaction completes, but obviously not with SELECTs.
Oracle is a funny beast. Like other gold-tooth-money databases it has some amazingly advanced features. But in other ways it is more 1980s than Top Gun crossed with Duran Duran.
It still doesn't have a native boolean type in SQL, for example. After 30 years.
