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https://issues.apache.org/jira/browse/LUCENE-7905?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16084128#comment-16084128
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Robert Muir commented on LUCENE-7905:
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Its a bit tricky to see the diffs since the file got moved too, but basically
it just replaces MultiTermsEnum with a standard PQ?
Do we know why this is a speedup? Is it an inefficiency in MultiTermsEnum?
> Optimizations for OrdinalMap
> ----------------------------
>
> Key: LUCENE-7905
> URL: https://issues.apache.org/jira/browse/LUCENE-7905
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Michael McCandless
> Assignee: Michael McCandless
> Fix For: 7.1
>
> Attachments: LUCENE-7905.patch
>
>
> {{OrdinalMap}} is a useful class to quickly map per-segment ordinals to
> global space, but it's fairly costly to build, which must typically be done
> on every NRT refresh.
> I'm using it quite heavily in two different places, one for
> {{SortedSetDocValuesFacetCounts}}, and another custom usage, and I found some
> small optimizations to improve its construction time.
> I switched it to use a simple priority queue to merge the terms instead of
> the more general {{MultiTermsEnum}}, which does extra work since it must also
> provide postings, implement seekExact, etc.
> I also pulled {{OrdinalMap}} out into its own oal.index class.
> When testing construction time for my case the patch is ~16% faster (159.9s
> -> 134.2s) in one case with 91.4 M terms and ~9% faster (115.6s -> 105.7s) in
> another case with 26.6 M terms.
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