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https://issues.apache.org/jira/browse/LUCENE-7304?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15302569#comment-15302569
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Martijn van Groningen commented on LUCENE-7304:
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bq. I wonder... instead couldn't we get a DocIdSetIterator of parent docs and
kind of intersect it with the child DISI?
I wondered that a while ago too, but we can't go backwards with
`DocIdSetIterator` and this what the advance method
('parentBits.prevSetBit(parentTarget-1)') requires of the block join query to
figure out where the first child starts for 'parentTarget'.
> 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_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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