J. Zhang, J. Tang, and J. Li. Advances in Databases: Concepts, Systems and Applications, volume 4443 of Lecture Notes in Computer Science, Springer Berlin Heidelberg, (2007)
E. Smirnova. Proceedings of the 34th International ACM SIGIR Conference on Research and Development in Information Retrieval, page 1191--1192. New York, NY, USA, ACM, (2011)