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https://issues.apache.org/jira/browse/LUCENE-7391?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15391441#comment-15391441
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Martijn van Groningen commented on LUCENE-7391:
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bq. +1 - freeze() was a hack, and I've been meaning to open an issue to make
things properly immutable for ages.
[~romseygeek] Then lets try to fix this in master :) I've opened LUCENE-7394 to
track this.
> 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,
> 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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