[Solr Wiki] Update of "CollapsingQParserPlugin" by CassandraTargett

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The "CollapsingQParserPlugin" page has been changed by CassandraTargett:
https://wiki.apache.org/solr/CollapsingQParserPlugin?action=diff&rev1=9&rev2=10

Comment:
add migration note

- '''Solr 4.6'''
+ {{{#!wiki important
+ This page exists for the Solr Community to share Tips, Tricks, and Advice about
+ [[https://cwiki.apache.org/confluence/display/solr/Collapse+and+Expand+Results|Collapsing Query Parser]].
+  
+ Reference material previously located on this page has been migrated to the
+ [[https://cwiki.apache.org/solr/|Official Solr Reference Guide]].
+ If you need help, please consult the Reference Guide for the version of Solr you are using
+ for the specific details about using [[https://cwiki.apache.org/confluence/display/solr/Collapse+and+Expand+Results|this feature]].
+  
+ If you'd like to share information about how you use this feature, please [[FrontPage#How_to_edit_this_Wiki|add it to this page]].
+ /* cwikimigrated */
+ }}}
  
- 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.
- 
- The CollapsingQParserPlugin collapses the result set to a single document per group before it forwards the result set to the rest of the search components. So all downstream components (faceting etc...) will work with the collapsed result set.
- 
- The CollapsingQParserPlugin has a lean set of features that allow the CollapsingQParserPlugin to perform well with millions of distinct groups in the result set.
- 
- '''Sample syntax:'''
- 
- Collapse based on the highest scoring document:
- 
-  . fq={!collapse field=field_name}
- 
- Collapse based on the min value of a numeric field:
- 
-  . fq={!collapse field=field_name min=field_name}
- 
- Collapse based on the max value of a numeric field:
- 
-  . fq={!collapse field=field_name max=field_name}
- 
- Collapse based on the min/max value of a function. The new  cscore() function can be used with the CollapsinqQParserPlugin to  return the  score of the current document being collapsed.  (Solr 4.7)
- 
-  fq={!collapse field=field_name max=sum(cscore(),field(A))}
- 
- Collapse with a null policy:
- 
-  . fq={!collapse field=field_name nullPolicy=nullPolicy}
- 
- 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 fully supports the QueryElevationComponent.
- 
- '''TODO''': Coming soon the ExpandComponent (SOLR-5270). The ExpandComponent expands the groups for a single page of results collapsed by the CollapsingQParserPlugin.
-