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https://issues.apache.org/jira/browse/LUCENE-7391?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15389705#comment-15389705
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David Smiley commented on LUCENE-7391:
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The test is fine; I thought the perf fix would also be in the patch. Let me
know when you have that and I'll review and commit later. Also... your .patch
appears to _not_ be a normal patch file.... see
https://wiki.apache.org/lucene-java/HowToContribute#Creating_a_patch
> MemoryIndexReader.fields() performance regression
> -------------------------------------------------
>
> Key: LUCENE-7391
> URL: https://issues.apache.org/jira/browse/LUCENE-7391
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Steve Mason
> Assignee: David Smiley
> Attachments: LUCENE-7391-test.patch, LUCENE-7391.patch
>
>
> While upgrading our codebase from Lucene 4 to Lucene 6 we found a significant
> performance regression - a 5x slowdown
> On profiling the code, the method MemoryIndexReader.fields() shows up as one
> of the hottest methods
> Looking at the method, it just creates a copy of the inner {{fields}} Map
> before passing it to {{MemoryFields}}. It does this so that it can filter out
> fields with {{numTokens <= 0}}.
> The simplest "fix" would be to just remove the copying of the map completely,
> and pass {{fields}} directly to {{MemoryFields}}. It's simple and removes
> any slowdown caused by this method. It does potentially change behaviour
> though, but none of the unit tests seem to test that behaviour so I wonder
> whether it's necessary (I looked at the original ticket LUCENE-7091 that
> introduced this code, I can't find much in way of an explanation). I'm going
> to attach a patch to this effect anyway and we can take things from there
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