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  1. Lucene - Core
  2. LUCENE-6789

change IndexSearcher default similarity to BM25

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    • Type: Improvement
    • Status: Resolved
    • Priority: Major
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 6.0
    • Component/s: None
    • Labels:
    • Lucene Fields:


      Since Lucene 4.0, the statistics needed for this are always present, so we can make the change without any degradation.

      I think the change should be a 6.0 change only: it will prevent any surprises. DefaultSimilarity is renamed to ClassicSimilarity to prevent confusion. No indexing change is needed as we use the same norm format, its just a runtime switch. Users can just do IndexSearcher.setSimilarity(new ClassicSimilarity()) to get the old behavior. I did not change solr's default here, I think that should be a separate issue, since it has more concerns: e.g. factories in configuration files and so on.

      One issue was the generation of synonym queries (posinc=0) by QueryBuilder (used by parsers). This is kind of a corner case (query-time synonyms), but we should make it nicer. The current code in trunk disables coord, which makes no sense for anything but the vector space impl. Instead, this patch adds a SynonymQuery which treats occurrences of any term as a single pseudoterm. With english wordnet as a query-time synonym dict, this query gives 12% improvement in MAP for title queries on BM25, and 2% with Classic (not significant). So its a better generic approach for synonyms that works with all scoring models.

      I wanted to use BlendedTermQuery, but it seems to have problems at a glance, it tries to "take on the world", it has problems like not working with distributed scoring (doesn't consult indexsearcher for stats). Anyway this one is a different, simpler approach, which only works for a single field, and which calls tf(sum) a single time.


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            • Assignee:
              rcmuir Robert Muir


              • Created:

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