it's not that simple

Yeah. Actually as far as I can tell you have two choices if you want 
transaction consistency: use multiversion concurrency control, or use 
locks. Locks have a *much* worse performance hit in the concurrent case, 
and annoy developers more, but a lower hit in the single-threaded case. 
It's a toss-up which to use :)

(Oracle moved to MVCC a while back, too, while keeping around a huge pile 
of grot to try to mollify people who were used to locking everything and 
messing around in horrible ways with 'rollback segments' and that crud.)