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https://issues.apache.org/jira/browse/LUCENE-7905?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16085139#comment-16085139
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Robert Muir commented on LUCENE-7905:
-------------------------------------
I think its good to pull out OrdinalMap into its own class from MultiDocValues,
but i think its a little trappy that it then has no warnings on it and just
some cool sounding javadocs.
MultiDocValues still warns you twice:
https://github.com/apache/lucene-solr/blob/master/lucene/core/src/java/org/apache/lucene/index/MultiDocValues.java#L40-L46
But I think we should have some kind of doc about the cost of this thing in
OrdinalMap now that its separated? The "tax" of multiple segments is real here,
makes it a hotspot just like it is for blocktree term dictionaries at merge.
> 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, LUCENE-7905.patch,
> LUCENE-7905-specialized.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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