Robert Muir created LUCENE-6789:
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Summary: change IndexSearcher default similarity to BM25
Key: LUCENE-6789
URL: https://issues.apache.org/jira/browse/LUCENE-6789
Project: Lucene - Core
Issue Type: Improvement
Reporter: Robert Muir
Fix For: 6.0
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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