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https://issues.apache.org/jira/browse/SOLR-5725?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15450784#comment-15450784
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David Smiley commented on SOLR-5725:
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{{facet.exists=true}} looks good to me... but it should be okay to not specify 
facet.method -- it will be enum to support this feature.  I'm cool with it 
being an error if for some reason you set it to something other than enum.

BTW this feature might theoretically be brought into the other facet types in 
the future.  e.g. {noformat}facet.query={! 
facet.exists=true}field:val{noformat}  or {{facet.range.exists=true}}

> Efficient facets without counts for enum method
> -----------------------------------------------
>
>                 Key: SOLR-5725
>                 URL: https://issues.apache.org/jira/browse/SOLR-5725
>             Project: Solr
>          Issue Type: Improvement
>          Components: search
>            Reporter: Alexey Kozhemiakin
>            Assignee: Mikhail Khludnev
>             Fix For: master (7.0), 6.3
>
>         Attachments: SOLR-5725-5x.patch, SOLR-5725-master.patch, 
> SOLR-5725.patch, SOLR-5725.patch, SOLR-5725.patch, SOLR-5725.patch, 
> SOLR-5725.patch
>
>
> Shot version:
> This improves performance for facet.method=enum when it's enough to know that 
> facet count>0, for example when you it's when you dynamically populate 
> filters on search form. New method checks if two bitsets intersect instead of 
> counting intersection size.
> Long version:
> We have a dataset containing hundreds of millions of records, we facet by 
> dozens of fields with many of facet-excludes and have relatively small number 
> of unique values in fields, around thousands.
> Before executing search, users work with "advanced search" form, our  goal is 
> to populate dozens of filters with values which are applicable with other 
> selected values, so basically this is a use case for facets with mincount=1, 
> but without need in actual counts.
> Our performance tests showed that facet.method=enum works much better than 
> fc\fcs, probably due to a specific ratio of "docset"\"unique terms count". 
> For example average execution of query time with method fc=1500ms, fcs=2600ms 
> and with enum=280ms. Profiling indicated the majority time for enum was spent 
> on intersecting docsets.
> Hers's a patch that introduces an extension to facet calculation for 
> method=enum. Basically it uses docSetA.intersects(docSetB) instead of 
> docSetA. intersectionSize (docSetB).
> As a result we were able to reduce our average query time from 280ms to 60ms.



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