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https://issues.apache.org/jira/browse/LUCENE-2454?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12889217#action_12889217
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Mark Harwood commented on LUCENE-2454:
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bq. I made a minor modification your approch by making it do a "Forward-scan" 
instead of reverse scan

Interesting, but I'm not sure what guarantees Lucene will make about:

* Sequencing of calls on scorer.nextDoc (i.e. are calls to all scorers involved 
guaranteed to be in doc-insert order ?)
* Index-time merging of segments (i.e. are all segments merged together in an 
order that keeps the parent doc in one segment next to the child doc from the 
next segment?)

That seems like a fragile set of dependencies. 
Also, don't things get tricky when reporting matches from NestedDocumentQuery 
and PerParentLimitedQuery back to the collector? During the query process the 
IndexSearcher resets the docId context (Collector.setNextReader) as it moves 
from one Scorer/segment to another. If we are delaying the assessment/reporting 
of matches until we've crossed a segment boundary it is too late to report a 
match on a child doc id from a previous segment as the collector has already 
changed context. Unfortunately "PerParentLimitedQuery" needs to do this when 
selecting the "best" children for a single parent.



> Nested Document query support
> -----------------------------
>
>                 Key: LUCENE-2454
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2454
>             Project: Lucene - Java
>          Issue Type: New Feature
>          Components: Search
>    Affects Versions: 3.0.2
>            Reporter: Mark Harwood
>            Assignee: Mark Harwood
>            Priority: Minor
>         Attachments: LuceneNestedDocumentSupport.zip
>
>
> A facility for querying nested documents in a Lucene index as outlined in 
> http://www.slideshare.net/MarkHarwood/proposal-for-nested-document-support-in-lucene

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