Nested Field Collapsing [LUCENE-3257]

Currently specifying grouping on multiple fields returns multiple datasets.

It would be nice if Solr supported cascading / nested grouping by applying the first group over the entire result set, the next over each group and so forth and so forth.

Even if limited to supporting nesting grouping 2 levels deep would cover alot of use cases.

group.field=location&group.field=type

-Location X
---Type 1
-----documents
---Type 2
----documents
-Location Y
---Type 1
----documents
---Type 2
----documents

instead of
-Location X
- documents
-Location Y
--documents

-Type 1
--documents
-Type2
--documents

See also https://issues.apache.org/jira/browse/SOLR-2553

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Migrated from [LUCENE-3257](https://issues.apache.org/jira/browse/LUCENE-3257) by Martijn Laarman, 6 votes

Start by reviewing the existing grouping behavior for repeated group.field parameters and the related SOLR-2553 issue. Define how the first group should contain nested results for each subsequent group, including the supported nesting depth and expected response structure; completion would require the documented behavior and coverage for the example hierarchy.

Written by the indexing model from the issue text.