I think it could be compelling if we restricted doc values to use an
iterator API at read time, instead of the more general random access
API we have today:
- It would make doc values disk usage more of a "you pay for what
what you actually use", like postings, which is a compelling
reduction for sparse usage.
- I think codecs could compress better and maybe speed up decoding
of doc values, even in the non-sparse case, since the read-time
API is more restrictive "forward only" instead of random access.
- We could remove getDocsWithField entirely, since that's
implicit in the iteration, and the awkward "return 0 if the
document didn't have this field" would go away.
- We can remove the annoying thread locals we must make today in
CodecReader, and close the trappy "I accidentally shared a
single XXXDocValues instance across threads", since an iterator is
inherently "use once".
- We could maybe leverage the numerous optimizations we've done for
postings over time, since the two problems ("iterate over doc ids
and store something interesting for each") are very similar.
This idea has come up many in the past, e.g.
LUCENE-7253 is a recent
example, and very early iterations of doc values started with exactly
However, it's a truly enormous change, likely 7.0 only. Or maybe we
could have the new iterator APIs also ported to 6.x side by side with
the deprecate existing random-access APIs.