PostgreSQL can use that optimization since an index scan in PostgreSQL still returns all tuples from the heap in index order, not in heap order. During an index

PostgreSQL can use that optimization since an index scan in PostgreSQL still returns all tuples from the heap in index order, not in heap order. During an index scan PostgreSQL traverses the B-tree in index order and for each matching tuple found in the index it fetches the data from the heap (which is a quite cheap O(1) operation).