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https://issues.apache.org/jira/browse/LUCENE-7407?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15409850#comment-15409850
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Adrien Grand commented on LUCENE-7407:
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Wow, thanks for getting this started! I had a quick look at the diff, it looks
great. One minor suggestion I have would be to make the API return a
DocIdSetIterator (like Scorer) rather than extend it. In LUCENE-6919 this
helped avoid having to wrap all the time, which also made performance better.
In the case of doc values, codecs could have one way to get an iterator that
would be reused by all doc value types (NUMERIC, SORTED, ...) for instance.
> 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
>
> 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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