1. A method of generating an annotated search index, the method executable at a server, the method comprising: a) retrieving a portion of a historical search session for a first search query, the portion including a first resource and a second resource, both the first and the second resource being relevant to the first search query, the first resource including at least some of the search terms from the first search query and having been indexed to the included search terms in a first search index, the second resource being associated with a reformulated query in the historical search session, the second resource having none of the search terms of the first search query and not having been indexed to the search terms in the first search index; b) generating an association parameter for the second resource, the association parameter being based on a first historical parameter and a second historical parameter, the first historical parameter being a number of transitions between the first resource and the second resource in the historical search session, and the second historical parameter being a prior user time spent interacting with the second resource in the historical search session; and, c) responsive to the association parameter for the second resource being above a predetermined threshold, associating the second resource with one or more of the first resource and the included search terms, thereby generating the annotated search index for the included search terms.