With the addition of term vectors, I think there are new ways that we can now do sorting in Lucene. It's different from boosting documents, but sometimes it is specifically sorting that is required. I've written HitCollectors that retrieve term vectors on each call to collect() and the performance remains quite acceptable (that's even without further optimizations to the term vectors). So it is now possible to write HitCollectors that retrieve termvectors for a specific field and sort documents they collect by values in those fields. If the field is a "keyword" it will only have a single term, which is probably most appropriate for sorting. The term numbers used by the term vectors are designed to be ordered by lexicographical ordering of the terms themselves. In other words, if number of term A is less than number of term B, then term A is also less than the term B. If the term text is choosen with thought, this will allow very efficient sorting, I think.
Perhaps sorting should be a property that is set on a query? It could be as simple as query.sortBy(String field, boolean ascending). The queyr will then sort by the specified field's first term. The field must be vectorized. This way it could influence the behavior of the default HitCollector (which now sorts only by score).
