[jira] [Created] (LUCENE-7391) MemoryIndexReader.fields() performance regression

["Steve Mason (JIRA)" <ji...@apache.org>]

Steve Mason created LUCENE-7391:
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             Summary: 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


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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