In this scheme, we process each posting ⌊T/n⌋ times because we touch it
during each of ⌊T/n⌋ merges where n is the size of the auxiliary index and T
the total number of postings. Thus, the overall time complexity is Θ(T
2/n).
(We neglect the representation of terms here and consider only the docIDs.
For the purpose of time complexity, a postings list is simply a list of docIDs.)