SpellCheck's Collation in Solr is a way to ensure spellcheck/suggestions
will actually net results (taking into account context like filtering).
In order to do this (from my understanding), it generates candidate queries,
executes them, and saves the total hit count: collation.setHits(hits).
For a large index it seems this might be doing too much work: in particular
I'm interested in ensuring this feature can work fast enough/well for autosuggesters.
So I think we should offer an 'approximate' mode that uses an early-terminating
Collector, collect()ing only N docs (e.g. n=1), and we approximate this result
count based on docid space.
I'm not sure what needs to happen on the solr side (possibly support for custom collectors?),
but I think this could help and should possibly be the default.