In TFSGs, unification itself is very costly. Indexing could be the key to efficient parsing by reducing the number of unifications while retrieving categories from the chart.
The major problem with indexing TFSGs lies in the large amount of information stored in a TFS. This makes indexing difficult for TFSG parsers, since the extraction of an index key from each category is not a trivial process. Another obstacle is that at least some of the index has to be built and maintained during parsing (unlike in databases, where the index is typically built off-line), since not all information in a TFS is available before parsing.
All these aspects of TFSG indexing, together with similar problems and their solutions in related areas, are presented in Section 4.4 and in Chapter 5.