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https://issues.apache.org/jira/browse/LUCENE-7304?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15304145#comment-15304145
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Martijn van Groningen commented on LUCENE-7304:
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bq. Does this approach work out to less than one bit per doc?
Unfortunately it is more than that. But with current block join implementation
the memory cost does increase (requires extra bit sets) when there are multiple
levels of parent-child relations, while with this approach the memory costs
remains the same (it just needs one numeric doc values field to encode the
multiple layers of document blocks).
bq. our doc values compression isn't THAT good yet
Maybe if doc values becomes an iterator based, then I guess with delta
encoding, we could get closer to 1 bit per doc?
> 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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