[jira] [Created] (SOLR-8057) Change default Sim to BM25 (w/backcompat config handling)

["Hoss Man (JIRA)" <ji...@apache.org>]

Hoss Man created SOLR-8057:
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             Summary: Change default Sim to BM25 (w/backcompat config handling)
                 Key: SOLR-8057
                 URL: https://issues.apache.org/jira/browse/SOLR-8057
             Project: Solr
          Issue Type: Task
            Reporter: Hoss Man
            Priority: Blocker
             Fix For: 6.0


LUCENE-6789 changed the default similarity for IndexSearcher to BM25 and renamed "DefaultSimilarity" to "ClassicSimilarity"

Solr needs to be updated accordingly:

* a "ClassicSimilarityFactory" should exist w/expected behavior/javadocs
* default behavior (in 6.0) when no similarity is specified in configs should (ultimately) use BM25 depending on luceneMatchVersion
** either by assuming BM25SimilarityFactory or by changing the internal behavior of DefaultSimilarityFactory
* comments in sample configs need updated to reflect new default behavior
* ref guide needs updated anywhere it mentions/implies that a particular similarity is used (or implies TF-IDF is used by default)



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