Michael McCandless commented on LUCENE-7407:

Hmm, in Lucene's nightly perf tests, the {{TermDateFacets}} got only a bit 
slower (~11%), not 40% slower.  Yonik, can you give more details on your 
benchmark so others can run it?

We should also look at the Solr faceting code to see if it can be improved on 
how it's using the DV iterators; I just did a quick cutover to the iterator API 
for this issue, and maybe there's something inefficient there.

I don't think Lucene should have two APIs (LUCENE-7462).  This will lead to too 
much bifurcation of the code that consumes doc values.  That's the wrong 
tradeoff, and we shouldn't let performance mess up our APIs that heavily.

That said, I think an {{advanceExact}} would be a good middle ground, if we 
show it can in fact help performance.  We could make a simple default impl in 
either DISI or maybe a new base class for all doc values impls.

> Explore switching doc values to an iterator API
> -----------------------------------------------
>                 Key: LUCENE-7407
>                 URL: https://issues.apache.org/jira/browse/LUCENE-7407
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>              Labels: docValues
>             Fix For: master (7.0)
>         Attachments: LUCENE-7407.patch
> 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
> this ;)
> 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.

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