I have changed the status of this proposal as rejected.
To resume the final state of this proposal there is no consensus on the interest to add a hook on start xact commands. Also the only useful case for this hook was to be able to have a server side automatic rollback at statement level. It can be regrettable because I don't think that PostgreSQL will have such feature before a long time (that's probably better) and a way to external implementation through an extension would be helpful for migration from other RDBMS like DB2 or Oracle. The only ways to have this feature is to handle the rollback at client side using savepoint, which is at least 3 times slower than a server side implementation, or not use such implementation at all. Outside not being performant it doesn't scale due to txid wraparound. And the last way is to use a proprietary forks of PostgreSQL, some are proposing this feature.
-- Gilles Darold http://www.darold.net/
