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https://issues.apache.org/jira/browse/LUCENE-3122?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13950284#comment-13950284
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Otis Gospodnetic commented on LUCENE-3122:
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Is this about grouping or about faceting (a la Solr pivot faceting?)
> Cascaded grouping
> -----------------
>
> Key: LUCENE-3122
> URL: https://issues.apache.org/jira/browse/LUCENE-3122
> Project: Lucene - Core
> Issue Type: Improvement
> Components: modules/grouping
> Reporter: Michael McCandless
> Labels: gsoc2014
> Fix For: 4.8
>
>
> Similar to SOLR-2526, in that you are grouping on 2 separate fields, but
> instead of treating those fields as a single grouping by a compound key, this
> change would let you first group on key1 for the primary groups and then
> secondarily on key2 within the primary groups.
> Ie, the result you get back would have groups A, B, C (grouped by key1) but
> then the documents within group A would be grouped by key 2.
> I think this will be important for apps whose documents are the product of
> denormalizing, ie where the Lucene document is really a sub-document of a
> different identifier field. Borrowing an example from LUCENE-3097, you have
> doctors but each doctor may have multiple offices (addresses) where they
> practice and so you index doctor X address as your lucene documents. In this
> case, your "identifier" field (that which "counts" for facets, and should be
> "grouped" for presentation) is doctorid. When you offer users search over
> this index, you'd likely want to 1) group by distance (ie, < 0.1 miles, < 0.2
> miles, etc., as a function query), but 2) also group by doctorid, ie cascaded
> grouping.
> I suspect this would be easier to implement than it sounds: the per-group
> collector used by the 2nd pass grouping collector for key1's grouping just
> needs to be another grouping collector. Spookily, though, that collection
> would also have to be 2-pass, so it could get tricky since grouping is sort
> of recursing on itself.... once we have LUCENE-3112, though, that should
> enable efficient single pass grouping by the identifier (doctorid).
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