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https://issues.apache.org/jira/browse/SOLR-11711?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16287988#comment-16287988
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ASF GitHub Bot commented on SOLR-11711:
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Github user HoustonPutman closed the pull request at:
https://github.com/apache/lucene-solr/pull/279
> distributed pivot & field facets can processes excessive docs unneccessarily
> due to internal mincount=0
> -------------------------------------------------------------------------------------------------------
>
> Key: SOLR-11711
> URL: https://issues.apache.org/jira/browse/SOLR-11711
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Components: faceting
> Affects Versions: master (8.0)
> Reporter: Houston Putman
> Assignee: Hoss Man
> Labels: pull-request-available
> Fix For: master (8.0), 7.3
>
>
> Currently while sending pivot facet requests to each shard, the
> {{facet.pivot.mincount}} is set to {{0}} if the facet is sorted by count with
> a specified limit > 0. However with a mincount of 0, the pivot facet will use
> exponentially more wasted memory for every pivot field added. This is because
> there will be a total of {{limit^(# of pivots)}} pivot values created in
> memory, even though the vast majority of them will have counts of 0, and are
> therefore useless.
> Imagine the scenario of a pivot facet with 3 levels, and
> {{facet.limit=1000}}. There will be a billion pivot values created, and there
> will almost definitely be nowhere near a billion pivot values with counts > 0.
> This likely due to the reasoning mentioned in [this comment in the original
> distributed pivot facet
> ticket|https://issues.apache.org/jira/browse/SOLR-2894?focusedCommentId=13979898].
> Basically it was thought that the refinement code would need to know that a
> count was 0 for a shard so that a refinement request wasn't sent to that
> shard. However this is checked in the code, [in this part of the refinement
> candidate
> checking|https://github.com/apache/lucene-solr/blob/releases/lucene-solr/7.1.0/solr/core/src/java/org/apache/solr/handler/component/PivotFacetField.java#L275].
> Therefore if the {{pivot.mincount}} was set to 1, the non-existent values
> would either:
> * Not be known, because the {{facet.limit}} was smaller than the number of
> facet values with positive counts. This isn't an issue, because they wouldn't
> have been returned with {{pivot.mincount}} set to 0.
> * Would be known, because the {{facet.limit}} would be larger than the number
> of facet values returned. therefore this conditional would return false
> (since we are only talking about pivot facets sorted by count).
> The solution, is to use the same pivot mincount as would be used if no limit
> was specified.
> This also relates to a similar problem in field faceting that was "fixed" in
> [SOLR-8988|https://issues.apache.org/jira/browse/SOLR-8988#13324]. The
> solution was to add a flag, {{facet.distrib.mco}}, which would enable not
> choosing a mincount of 0 when unnessesary. Since this flag can only increase
> performance, and doesn't break any queries I have removed it as an option and
> replaced the code to use the feature always.
> There was one code change necessary to fix the MCO option, since the
> refinement candidate selection logic had a bug. The bug only occured with a
> minCount > 0 and limit > 0 specified. When a shard replied with less than the
> limit requested, it would assume the next maximum count on that shard was the
> {{mincount}}, where it would actually be the {{mincount-1}} (because a facet
> value with a count of mincount would have been returned). Therefore the MCO
> didn't cause any errors, but with a mincount of 1 the refinement logic always
> assumed that the shard had more values with a count of 1.
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