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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