Other types of indexes

This chapter only describes construction of non-positional indexes. Except for the much larger data volume we need to accommodate, the main difference for positional indexes is that (termID, docID, (position1, position2, ...)) triples instead of (termID, docID) pairs have to be processed and that tokens and postings contain positional information in addition to docIDs. With this change, the algorithms discussed here can all be applied to positional indexes.

In the indexes we have considered so far, postings lists are ordered with respect to docID. As we will see in the next chapter, this is advantageous for compression - instead of docIDs we can compress smaller gaps between IDs, thus reducing space requirements for the index. However, this structure for the index is not optimal when we build ranked (Chapters 6 7 ) - as opposed to 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 impactordered, the insertion can occur anywhere, thus complicating the update of the inverted index.

Security is an important consideration for retrieval systems in corporations. A low-level employee should not be able to find the salary roster of the corporation, but authorized managers need to be able to search for it. Users' results lists must not contain documents they are barred from opening since the very existence of a document can be sensitive information.

A user-document matrix for access control lists. Element $(i,j)$ is 1 if user $i$ has access to document $j$ and 0 otherwise. During query processing, a user's access postings list is intersected with the results list returned by the text part of the index.

User authorization is often mediated through access control lists or ACLs. ACLs can be dealt with in an information retrieval system by representing each document as the set of users that can access them (Figure 4.8 ) and then inverting the resulting user-document matrix. The inverted ACL index has, for each user, a ``postings list'' of documents they can access - the user's access list. Search results are then intersected with this list. However, such an index is difficult to maintain when access permissions change - we discussed these difficulties in the context of incremental indexing for regular postings lists in the last section. It also requires the processing of very long postings lists for users with access to large document subsets. User membership is therefore often verified by retrieving access information directly from the file system at query time - even though this slows down retrieval.

We discussed indexes for storing and retrieving terms (as opposed to documents) in Chapter 3 .

Exercises.

• Can spelling correction compromise document-level security? Consider the case where a spelling correction is based on documents the user does not have access to.