I found the following discussion very helpful from back in 2012. http://markmail.org/thread/lkl7ffi77w7hpv6n
Probably the best description I've seen for how facets are actually calculated in Solr Cloud. Thanks. I presume this is for the most part still accurate. But, I have a slightly different question. Instead of ordering the results for a facet by count, what if I choose to order my results based on index. Will the counts still be correct in Solr Cloud? I would assume the same magic (as was described in the above link) could have been as easily applied to this approach. I assume that I could apply a prefix to the facet when ordering by index (such as 'A' to identify those products beginning with an 'A') and that the counts would still be correct. Thanks. Darin.