Major Description
Today the primary key lookup in lucene is not that great for systems like solr and elasticsearch that have versioning in front of IndexWriter.
To some extend BlockTree can "sometimes" help avoid seeks by telling you the term does not exist for a segment. But this technique (based on FST prefix) is fragile. The only other choice today is bloom filters, which use up huge amounts of memory.
I don't think we are using everything we know: particularly the version semantics.
Instead, if the FST for the terms index used an algebra that represents the max version for any subtree, we might be able to answer that there is no term T with version < V in that segment very efficiently.
Also ID fields dont need postings lists, they dont need stats like docfreq/totaltermfreq, etc this stuff is all implicit.
As far as API, i think for users to provide "IDs with versions" to such a PF, a start would to set a payload or whatever on the term field to get it thru indexwriter to the codec. And a "consumer" of the codec can just cast the Terms to a subclass that exposes the FST to do this version check efficiently.