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https://issues.apache.org/jira/browse/SOLR-6581?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15392392#comment-15392392
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Joel Bernstein commented on SOLR-6581:
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I just ran a test with collapse on trunk and validated [~dsmiley] findings.
Collapse at this point is so slow both with and without top_fc that it's really
not fit for purpose.
I'll dig a little to see if I can see what's happening.
> 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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