[jira] [Resolved] (LUCENE-6789) change IndexSearcher default similarity to BM25

["Robert Muir (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/LUCENE-6789?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Robert Muir resolved LUCENE-6789.
---------------------------------
    Resolution: Fixed

Thanks Adrien: I applied those fixes.

> 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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