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Yonik Seeley recently implemented a new method for faceting which will be available in Solr 1.4 (yet to be released). It is optimized for faceting on multi-valued fields with large number of unique terms but relatively low number of terms per document. The new method has made a large improvement in performance for faceted search and has cut memory usage at the same time.
Solr is an open source enterprise search server based on the Lucene Java search library, with XML/HTTP and JSON APIs, hit highlighting, faceted search, caching, replication, a web administration interface and many more features. It runs in a Java servlet
Public Websites using Solr
Although Lucene provides the ability to create your own queries through its API, it also provides a rich query language through the Query Parser, a lexer which interprets a string into a Lucene Query using JavaCC.
This page provides the Query Parser syntax in Lucene 1.9. If you are using a different version of Lucene, please consult the copy of docs/queryparsersyntax.html that was distributed with the version you are using.