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https://issues.apache.org/jira/browse/LUCENE-7905?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16084134#comment-16084134
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Adrien Grand commented on LUCENE-7905:
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I like the idea of switching to a simple priority queue to remove overhead.
Maybe we should also check how much better things would be with a specialized
priority queue too? As far as I remember, it helped a lot with disjunction
scorers.
One minor thing that confuses me with the patch is that it moves off
{{MultiTermsEnum}} but reuses {{MultiTermsEnum.TermsEnumIndex}} and adds an
additional method to it. Maybe we should decouple {{OrdinalMap}} and
{{MultiTermsEnum}} entirely and give {{OrdinalMap}} its own {{TermsEnum}}+index
wrapper?
> 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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