Cascaded grouping
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Key: LUCENE-3122
URL: https://issues.apache.org/jira/browse/LUCENE-3122
Project: Lucene - Java
Issue Type: Improvement
Components: modules/grouping
Reporter: Michael McCandless
Fix For: 3.2, 4.0
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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