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https://issues.apache.org/jira/browse/LUCENE-6789?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Robert Muir updated LUCENE-6789:
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Attachment: LUCENE-6789.patch
> 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
>
> Attachments: LUCENE-6789.patch
>
>
> 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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