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https://issues.apache.org/jira/browse/LUCENE-1634?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12708995#action_12708995
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John Wang commented on LUCENE-1634:
-----------------------------------
The current lucene implementation, optimize(int) selects segments to merge
based on the file size of the segment file: say the index has 10 segments, and
optmize(6) is called, Lucene finds 4 smallest segments by number of bytes in
the segment files.
This selection criteria is flawed because you can have a very large segment in
terms of bytes but very small in terms of numDocs (if many deleted docs).
Having these segment files around impacts performance considerably.
This is what this patch is trying to fix this in a non-intrusive manner by
extending the LogMergePolicy and by normalizing the calculation of the segment
size including the delete count.
> LogMergePolicy should use the number of deleted docs when deciding which
> segments to merge
> ------------------------------------------------------------------------------------------
>
> Key: LUCENE-1634
> URL: https://issues.apache.org/jira/browse/LUCENE-1634
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Index
> Reporter: Yasuhiro Matsuda
> Assignee: Michael McCandless
> Priority: Minor
> Fix For: 2.9
>
> Attachments: LUCENE-1634.patch
>
>
> I found that IndexWriter.optimize(int) method does not pick up large segments
> with a lot of deletes even when most of the docs are deleted. And the
> existence of such segments affected the query performance significantly.
> I created an index with 1 million docs, then went over all docs and updated a
> few thousand at a time. I ran optimize(20) occasionally. What saw were large
> segments with most of docs deleted. Although these segments did not have
> valid docs they remained in the directory for a very long time until more
> segments with comparable or bigger sizes were created.
> This is because LogMergePolicy.findMergeForOptimize uses the size of segments
> but does not take the number of deleted documents into consideration when it
> decides which segments to merge. So, a simple fix is to use the delete count
> to calibrate the segment size. I can create a patch for this.
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