Compaction techniques for nextword indexes

Most queries to text search engines are ranked or Boolean. Phrase querying is a powerful technique for rening searches, but is expensive to implement on conventional indexes. In other work, a nextword index has been proposed as a structure specically designed for phrase queries. Nextword indexes are, however, relatively large. In this paper we introduce new compaction techniques for nextword indexes. In contrast to most index compression schemes, these techniques are lossy, yet as we show allow full resolution of phrase queries without false match checking. We show experimentally that our novel techniques lead to signican t savings in index size.