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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] [Resolved] (LUCENE-3029) MultiPhraseQuery assigns different scores to identical docs when using 0 pos-incr
Date Mon, 02 May 2011 20:41:03 GMT

     [ https://issues.apache.org/jira/browse/LUCENE-3029?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Michael McCandless resolved LUCENE-3029.
----------------------------------------

    Resolution: Fixed

> MultiPhraseQuery assigns different scores to identical docs when using 0 pos-incr
> ---------------------------------------------------------------------------------
>
>                 Key: LUCENE-3029
>                 URL: https://issues.apache.org/jira/browse/LUCENE-3029
>             Project: Lucene - Java
>          Issue Type: Bug
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>            Priority: Minor
>             Fix For: 3.0.4, 3.2, 4.0
>
>         Attachments: LUCENE-3029.patch
>
>
> If you have two identical docs with tokens a b c all zero pos-incr (ie
> they occur on the same position), and you run a MultiPhraseQuery with
> [a, b] and [c] (all pos incr 0)... then the two docs will get
> different scores despite being identical.
> Admittedly it's a strange query... but I think the scorer ought to
> count the phrase as having tf=1 for each doc.
> The problem is that we are missing a tie-breaker for the PhraseQuery
> used by ExactPhraseScorer, and so the PQ ends up flip/flopping such
> that every other document gets the same score.  Ie, even docIDs all
> get one score and odd docIDs all get another score.
> Once I added the hard tie-breaker (ord) the scores are the same.
> However... there's a separate bug, that can over-count the tf, such
> that if I create the MPQ like this:
> {noformat}
>   mpq.add(new Term[] {new Term("field", "a")}, 0);
>   mpq.add(new Term[] {new Term("field", "b"), new Term("field", "c")}, 0);
> {noformat}
> I get tf=2 per doc, but if I create it like this:
> {noformat}
>   mpq.add(new Term[] {new Term("field", "b"), new Term("field", "c")}, 0);
>   mpq.add(new Term[] {new Term("field", "a")}, 0);
> {noformat}
> I get tf=1 (which I think is correct?).
> This happens because MultipleTermPositions freely returns the same
> position more than once: it just unions the positions of the two
> streams, so when both have their term at pos=0, you'll get pos=0
> twice, which is not good and leads to over-counting tf.
> Unfortunately, I don't see a performant way to fix that... and I'm not
> sure that it really matters that much in practice.

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