One other point here is that it's not really clear to me what a randomly
varying IV is supposed to accomplish. Surely we're not intending that
it prevents somebody from crafting a data set that causes bad hash
performance. If a user with DB access wants to cause a performance
problem, there are and always will be plenty of other avenues to making
that happen. If the idea is that for a data set that otherwise would have
bad hash performance, choosing a different IV would (almost always) fix
it, that sounds good but you're ignoring the inverse case: for a data set
that works fine, there would be some choices of IV that create a problem
where there was none before. I see no reason to think that the
probability of the former kind of situation is higher than the latter.
So I'm on board with using the extended hash functions when available,
but I'm not convinced that a varying IV buys us anything but trouble.
regards, tom lane