Unfortunately, the one-file-per-postings-list scheme is infeasible because
most file systems cannot efficiently handle very large numbers of files. The
simplest alternative is to store the index as one large file, that is, as a concatenation
of all postings lists. In reality, we often choose a compromise between
the two extremes (Section 4.7). To simplify the discussion, we choose the
simple option of storing the index as one large file here