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https://issues.apache.org/jira/browse/SOLR-6168?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14036879#comment-14036879
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Umesh Prasad commented on SOLR-6168:
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Also, it might be difficult to replace an ordered listing of comparators (which
is what SortFields boil down to) with a value source which has to provide an
absolute value for each document.
Under the hood, FieldValueCollapse actually uses a comparator for
collapsing, which reads its value from a single source. (score/field/function).
If collapsing comparator is composed out of multiple sub comparators (taken
from sort spec), then min/max can default to sort field itself.
This will avoid translating sort, which is essentially a series of
simple field/function based comparator, to a single valued consistent
function. (Not an easy task). Plus comparator based sort is more intuitive.
> CollapsingQParserPlugin ranks incorrectly when 3 or more sort params are used
> -----------------------------------------------------------------------------
>
> Key: SOLR-6168
> URL: https://issues.apache.org/jira/browse/SOLR-6168
> Project: Solr
> Issue Type: Bug
> Affects Versions: 4.7.1, 4.8.1
> Reporter: Umesh Prasad
> Assignee: Joel Bernstein
> Attachments: SOLR-6168-group-head-inconsistent-with-sort.patch
>
>
> CollapsingQParser Plugin ranks documents incorrectly when more than 2 sort
> fields are used.
> I have attached a test case, which demonstrates the broken behavior when 3
> sort fields are used.
> The failing test case patch is against Lucene/Solr 4.8.1 revision number
> 1603061
> PS : SOLR-5408 fixed the issue with sorting only for two sort fields, by
> allowing one to specify max/min=<field-name>. However that requires 2nd sort
> field to be a numeric field. It will not work with string field or function
> query sort.
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