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 Section 4.5. 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.