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https://issues.apache.org/jira/browse/SOLR-5027?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13838010#comment-13838010
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Gabe Enslein commented on SOLR-5027:
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Hey Joel,
After reviewing the functionality as an alternative to using ngroups for
performance reasons, I have a use case that needs sorting specified in the
search to be respected before collapsing. I have documents that can have the
same score but when collapsing is performed, this only takes the first received
document. Many cases this document is less important or relevant and would
normally be lower in results if sorting pre-collapse was respected. Is there a
possibility that something could be done to either adapt this functionality to
respect sorting before collapsing?
> Field Collapsing PostFilter
> ---------------------------
>
> Key: SOLR-5027
> URL: https://issues.apache.org/jira/browse/SOLR-5027
> Project: Solr
> Issue Type: New Feature
> Components: search
> Affects Versions: 5.0
> Reporter: Joel Bernstein
> Assignee: Joel Bernstein
> Priority: Minor
> Fix For: 4.6, 5.0
>
> Attachments: SOLR-5027.patch, SOLR-5027.patch, SOLR-5027.patch,
> SOLR-5027.patch, SOLR-5027.patch, SOLR-5027.patch, SOLR-5027.patch,
> SOLR-5027.patch, SOLR-5027.patch
>
>
> This ticket introduces the *CollapsingQParserPlugin*
> The *CollapsingQParserPlugin* is a PostFilter that performs field collapsing.
> This is a high performance alternative to standard Solr field collapsing
> (with *ngroups*) when the number of distinct groups in the result set is high.
> For example in one performance test, a search with 10 million full results
> and 1 million collapsed groups:
> Standard grouping with ngroups : 17 seconds.
> CollapsingQParserPlugin: 300 milli-seconds.
> Sample syntax:
> Collapse based on the highest scoring document:
> {code}
> fq=(!collapse field=<field_name>}
> {code}
> Collapse based on the min value of a numeric field:
> {code}
> fq={!collapse field=<field_name> min=<field_name>}
> {code}
> Collapse based on the max value of a numeric field:
> {code}
> fq={!collapse field=<field_name> max=<field_name>}
> {code}
> Collapse with a null policy:
> {code}
> fq={!collapse field=<field_name> nullPolicy=<null_policy>}
> {code}
> There are three null policies:
> ignore : removes docs with a null value in the collapse field (default).
> expand : treats each doc with a null value in the collapse field as a
> separate group.
> collapse : collapses all docs with a null value into a single group using
> either highest score, or min/max.
> The CollapsingQParserPlugin also fully supports the QueryElevationComponent
> *Note:* The July 16 patch also includes and ExpandComponent that expands the
> collapsed groups for the current search result page. This functionality will
> be moved to it's own ticket.
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