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https://issues.apache.org/jira/browse/LUCENE-3503?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Robert Muir resolved LUCENE-3503.
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Resolution: Fixed
> DisjunctionSumScorer gives slightly (float iotas) different scores when you
> .nextDoc vs .advance
> ------------------------------------------------------------------------------------------------
>
> Key: LUCENE-3503
> URL: https://issues.apache.org/jira/browse/LUCENE-3503
> Project: Lucene - Java
> Issue Type: Bug
> Reporter: Michael McCandless
> Assignee: Robert Muir
> Fix For: 3.5, 4.0
>
> Attachments: LUCENE-3503.patch, LUCENE-3503.patch, LUCENE-3503.patch
>
>
> Spinoff from LUCENE-1536.
> I dug into why we hit a score diff when using luceneutil to benchmark
> the patch.
> At first I thought it was BS1/BS2 difference, but because of a bug in
> the patch it was still using BS2 (but should be BS1) -- Robert's last
> patch fixes that.
> But it's actually a diff in BS2 itself, whether you next or advance
> through the docs.
> It's because DisjunctionSumScorer, when summing the float scores for a
> given doc that matches multiple sub-scorers, might sum in a different
> order, when you had .nextDoc'd to that doc than when you had .advance'd
> to it.
> This in turn is because the PQ used by that scorer (ScorerDocQueue)
> makes no effort to break ties. So, when the top N scorers are on the
> same doc, the PQ doesn't care what order they are in.
> Fixing ScorerDocQueue to break ties will likely be a non-trivial perf
> hit, though, so I'm not sure whether we should do anything here...
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