This works in reverse, too, since we are unlikely to
choose a retrieval model that has no efficient query processing algorithm. Since
we will not be discussing retrieval models in detail until Chapter 7, we start this
chapter by describing an abstract model of ranking that motivates our choice of
indexes. After that, there are four main parts to the chapter. In the first part, we
discuss the different types of inverted index and what information about documents
is captured in each index.