Currently, StatelessSession.upsert does not result in FeatureNotSupportedException/SQLFeatureNotSupportedException, instead it behaves incorrectly:

1. It seem to result in executing two operations one after the other: at first update, then insert.
2. update has field values associated with wrong field names, which also means that the _id filter for the update operation has wrong value.
3. When upserting a document with a non-existing _id, it is successfully inserted. However, the aforementioned update behavior may accidentally corrupt an unrelated document it if happens to have _id equal to whatever value gets used in the update filter.
4. When upserting a document with a pre-existing _id, upsertion fails because insert fails. Though the reported error is misleading for the reasons reported in HIBERNATE-95.