Optimize facets when actually single valued?

[Robert Muir]

I am guessing at times people are lazy about schema definition. But, I
think with lucene 4 stats we can detect if a field is actually single
valued... Something like terms.size == terms.doccount == terms.sumdocfreq.
I have to think about it a bit, maybe its even simpler than this? Anyway,
this couple be used instead of actual schema def to just build a fieldcache
instead of uninverted field I think... Should be a simple opto but maybe
potent...