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https://issues.apache.org/jira/browse/LUCENE-3112?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13034730#comment-13034730
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Robert Muir commented on LUCENE-3112:
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We should really think through the consequences of this though.
If core features of lucene become implemented in a way that they rely upon
these sequential docids, we then lock ourselves out of future optimizations
such as reordering docids for optimal index compression.
> Add IW.add/updateDocuments to support nested documents
> ------------------------------------------------------
>
> Key: LUCENE-3112
> URL: https://issues.apache.org/jira/browse/LUCENE-3112
> Project: Lucene - Java
> Issue Type: Improvement
> Reporter: Michael McCandless
> Assignee: Michael McCandless
> Priority: Minor
> Fix For: 3.2, 4.0
>
> Attachments: LUCENE-3112.patch
>
>
> I think nested documents (LUCENE-2454) is a very compelling addition
> to Lucene. It's also a popular (many votes) issue.
> Beyond supporting nested document querying, which is already an
> incredible addition since it preserves the relational model on
> indexing normalized content (eg, DB tables, XML docs), LUCENE-2454
> should also enable speedups in grouping implementation when you group
> by a nested field.
> For the same reason, it can also enable very fast post-group facet
> counting impl (LUCENE-3097) when you what to
> count(distinct(nestedField)), instead of unique documents, as your
> "identifier". I expect many apps that use faceting need this ability
> (to count(distinct(nestedField)) not distinct(docID)).
> To support these use cases, I believe the only core change needed is
> the ability to atomically add or update multiple documents, which you
> cannot do today since in between add/updateDocument calls a flush (eg
> due to commit or getReader()) could occur.
> This new API (addDocuments(Iterable<Document>), updateDocuments(Term
> delTerm, Iterable<Document>) would also further guarantee that the
> documents are assigned sequential docIDs in the order the iterator
> provided them, and that the docIDs all reside in one segment.
> Segment merging never splits segments apart, so this invariant would
> hold even as merges/optimizes take place.
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