The comment is effectively explaining that we don't want to make the array big enough so that a malloc will always be required.
Doesn't what you are saying contradict both the formula (the +1 post-division) and the comments:
/* * Initial size of array must be more than ALLOCSET_SEPARATE_THRESHOLD; * see comments in grow_memtuples(). */ state->memtupsize = INITIAL_MEMTUPSIZE; state->memtuples = NULL;
and in grow_memtuples:
* That shouldn't happen because we chose the * initial array size large enough to ensure that palloc will be treating * both old and new arrays as separate chunks.
