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From Christian Kohlsch├╝tter (JIRA) <>
Subject [jira] Created: (LUCENE-954) Toggle score normalization in Hits
Date Mon, 09 Jul 2007 16:53:04 GMT
Toggle score normalization in Hits

                 Key: LUCENE-954
             Project: Lucene - Java
          Issue Type: Improvement
          Components: Search
    Affects Versions: 2.2
         Environment: any
            Reporter: Christian Kohlsch├╝tter
             Fix For: 2.2

The current implementation of the "Hits" class sometimes performs score normalization.
In particular, whenever the top-ranked score is bigger than 1.0, it is normalized to a maximum
of 1.0.

In this case, Hits may return different score results than TopDocs-based methods.

In my scenario (a federated search system), Hits delievered just plain wrong results.
I was merging results from several sources, all having homogeneous statistics (similar to
MultiSearcher, but over the Internet using HTTP/XML-based protocols).
Sometimes, some of the sources had a top-score greater than 1, so I ended up with garbled

I suggest to add a switch to enable/disable this score-normalization at runtime.
My patch (attached) has an additional peformance benefit, since score normalization now occurs
only when Hits#score() is called, not when creating the Hits result list. Whenever scores
are not required, you save one multiplication per retrieved hit (i.e., at least 100 multiplications
with the current implementation of Hits).

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