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https://issues.apache.org/jira/browse/SOLR-6581?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15392478#comment-15392478
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David Smiley commented on SOLR-6581:
------------------------------------

This is just one shard; the actual total collection is sharded beyond the 
levels most people shard.

In any case, maybe some time I'll peek into profiling what's going on to see if 
I can find any insights.  But that's low priority as our Solr 6 upgrade without 
the hint=top_fc is already a huge improvement overall.

> Efficient DocValues support and numeric collapse field implementations for 
> Collapse and Expand
> ----------------------------------------------------------------------------------------------
>
>                 Key: SOLR-6581
>                 URL: https://issues.apache.org/jira/browse/SOLR-6581
>             Project: Solr
>          Issue Type: Bug
>            Reporter: Joel Bernstein
>            Assignee: Joel Bernstein
>            Priority: Minor
>             Fix For: 5.0, 6.0
>
>         Attachments: SOLR-6581.patch, SOLR-6581.patch, SOLR-6581.patch, 
> SOLR-6581.patch, SOLR-6581.patch, SOLR-6581.patch, SOLR-6581.patch, 
> SOLR-6581.patch, SOLR-6581.patch, SOLR-6581.patch, SOLR-6581.patch, 
> SOLR-6581.patch, SOLR-6581.patch, SOLR-6581.patch, SOLR-6581.patch, 
> renames.diff
>
>
> The 4x implementation of the CollapsingQParserPlugin and the ExpandComponent 
> are optimized to work with a top level FieldCache. Top level FieldCaches have 
> a very fast docID to top-level ordinal lookup. Fast access to the top-level 
> ordinals allows for very high performance field collapsing on high 
> cardinality fields. 
> LUCENE-5666 unified the DocValues and FieldCache api's so that the top level 
> FieldCache is no longer in regular use. Instead all top level caches are 
> accessed through MultiDocValues. 
> This ticket does the following:
> 1) Optimizes Collapse and Expand to use MultiDocValues and makes this the 
> default approach when collapsing on String fields
> 2) Provides an option to use a top level FieldCache if the performance of 
> MultiDocValues is a blocker. The mechanism for switching to the FieldCache is 
> a new "hint" parameter. If the hint parameter is set to "top_fc" then the 
> top-level FieldCache would be used for both Collapse and Expand.
> Example syntax:
> {code}
> fq={!collapse field=x hint=TOP_FC}
> {code}
> 3)  Adds numeric collapse field implementations.
> 4) Resolves issue SOLR-6066
>  



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