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https://issues.apache.org/jira/browse/LUCENE-7304?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15318393#comment-15318393
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Martijn van Groningen commented on LUCENE-7304:
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bq. The last time I tried doc values, I could not use advance(target) on them.
Is that still the case?
That is still the case. But the way the doc value block join work is by storing
offsets (how far away is the first child doc in docids and how far away is the
closest parent) and at query time that is being used to advance the child
scorer. However when doc values become iterator based these offsets can be
encoded much more efficiently then is now the case.
> Doc values based block join implementation
> ------------------------------------------
>
> Key: LUCENE-7304
> URL: https://issues.apache.org/jira/browse/LUCENE-7304
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Martijn van Groningen
> Priority: Minor
> Attachments: LUCENE-5092-20140313.patch, LUCENE-7304-20160531.patch,
> LUCENE-7304-20160606.patch, LUCENE_7304.patch
>
>
> At query time the block join relies on a bitset for finding the previous
> parent doc during advancing the doc id iterator. On large indices these
> bitsets can consume large amounts of jvm heap space. Also typically due the
> nature how these bitsets are set, the 'FixedBitSet' implementation is used.
> The idea I had was to replace the bitset usage by a numeric doc values field
> that stores offsets. Each child doc stores how many docids it is from its
> parent doc and each parent stores how many docids it is apart from its first
> child. At query time this information can be used to perform the block join.
> I think another benefit of this approach is that external tools can now
> easily determine if a doc is part of a block of documents and perhaps this
> also helps index time sorting?
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