If a doc is listed as Book>Fiction>Sci-FI, it must be searchable when your facet query is "Book", "Book>Fiction", "Book>Fiction>Sci-fi".
I searched the Internet and the methods I came up with are:
1 Split the "Book>Fiction>Sci-FI" into "Book", "Book>Fiction", "Book>Fiction>Sci-fi" at index-time itself and store them in same field.
2 Have separate fields cat, sub_cat, sub_sub_cat. Split the catogories into corresponding fields and use Pivot facets http://wiki.apache.org/solr/SimpleFacetParameters#Pivot_.28ie_Decision_Tree.29_Faceting
What are other ways to do hierarchial facets, so that they can support any complex queries?