[jira] [Commented] (LUCENE-7407) Explore switching doc values to an iterator API

["ASF subversion and git services (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/LUCENE-7407?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15549552#comment-15549552 ] 

ASF subversion and git services commented on LUCENE-7407:
---------------------------------------------------------

Commit 001a3ca55b30656e0e42f612d927a7923f5370e9 in lucene-solr's branch refs/heads/master from Mike McCandless
[ https://git-wip-us.apache.org/repos/asf?p=lucene-solr.git;h=001a3ca ]

LUCENE-7407: speed up iterating norms a bit by having default codec implement the iterator directly


> 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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