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https://issues.apache.org/jira/browse/LUCENE-2680?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12919549#action_12919549
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Jason Rutherglen commented on LUCENE-2680:
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Maybe we should implement this as pending deletes per segment rather than using
a generational system because with LUCENE-2655, we may need to maintain the per
query/term docidupto per segment. The downside is the extraneous memory
consumed by the hash map, however, if we use BytesRefHash this'll be reduced,
or would it? Because we'd be writing the term bytes to a unique byte pool per
segment? Hmm... Maybe there's a more efficient way.
> Improve how IndexWriter flushes deletes against existing segments
> -----------------------------------------------------------------
>
> Key: LUCENE-2680
> URL: https://issues.apache.org/jira/browse/LUCENE-2680
> Project: Lucene - Java
> Issue Type: Improvement
> Reporter: Michael McCandless
> Fix For: 4.0
>
>
> IndexWriter buffers up all deletes (by Term and Query) and only
> applies them if 1) commit or NRT getReader() is called, or 2) a merge
> is about to kickoff.
> We do this because, for a large index, it's very costly to open a
> SegmentReader for every segment in the index. So we defer as long as
> we can. We do it just before merge so that the merge can eliminate
> the deleted docs.
> But, most merges are small, yet in a big index we apply deletes to all
> of the segments, which is really very wasteful.
> Instead, we should only apply the buffered deletes to the segments
> that are about to be merged, and keep the buffer around for the
> remaining segments.
> I think it's not so hard to do; we'd have to have generations of
> pending deletions, because the newly merged segment doesn't need the
> same buffered deletions applied again. So every time a merge kicks
> off, we pinch off the current set of buffered deletions, open a new
> set (the next generation), and record which segment was created as of
> which generation.
> This should be a very sizable gain for large indices that mix
> deletes, though, less so in flex since opening the terms index is much
> faster.
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