sion – instead of docIDs we can compress smaller gaps between IDs, thus
reducing space requirements for the index. However, this structure for the
RANKED index is not optimal when we build ranked (Chapters 6 and 7) – as opposed to
RETRIEVAL SYSTEMS Boolean – retrieval systems. In ranked retrieval, postings are often ordered according
to weight or impact, with the highest-weighted postings occurring
first. With this organization, scanning of long postings lists during query
processing can usually be terminated early when weights have become so
small that any further documents can be predicted to be of low similarity
to the query (see Chapter 6). In a docID-sorted index, new documents are
always inserted at the end of postings lists. In an impact-sorted index (Section
7.1.5, page 140), the insertion can occur anywhere, thus complicating the
update of the inverted index.