I wrote:
> You have the unique-versus-not dimension,
On second thought, actually not. What we want to look at is the penalty
for false matches due to *distinct* key values that happen to have the
same hash codes. Your test case for all-the-same is using all the same
key values, which means it'll hit the heap a lot, but none of those will
be wasted trips.
So what we need for testing is a few different key values that hash to
the same code. Not sure about an easy way to find such.
regards, tom lane