Lucene facet
http://itindex.net/detail/52925-lucene-%E7%B3%BB%E5%88%97-facet
http://www.cnblogs.com/forfuture1978/archive/2011/09/10/2172918.html
http://www.ibm.com/developerworks/cn/java/j-solr-lucene/
http://lucene.apache.org/core/3_6_1/api/contrib-facet/org/apache/lucene/facet/doc-files/userguide.html#indexed_facet_info
http://oak.cs.ucla.edu/cs144/projects/lucene/