about lucene grouping and facet history

http://www.searchworkings.org/blog/-/blogs/result-grouping-field-collapsing-with-solr/

alsk named as  Field Collapsing 

this is different with facet,

when you first glance ,you'll problely confused about this two concept

http://www.searchworkings.org/blog/-/blogs/search-result-grouping-field-collapsing-in-lucene-solr/

http://stackoverflow.com/questions/8550818/whats-the-difference-between-grouping-and-facet-in-lucene-3-5