Hmm ... there's another point here, which is that the array size creates
a hard maximum on the number of entries, whereas the hash table was a
bit more forgiving. What is the proof that the array won't overflow?
The fact that the equivalent data structure on the master can't hold
more than this many entries doesn't seem to me to prove that, because
we will add intermediate not-observed XIDs to the array.
regards, tom lane
