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

• Type: Task
• Status: Resolved
• Priority: Blocker
• Resolution: Fixed
• Fix Version/s: 6.0
• Assignee: Chris M. Hostetter
• Reporter: Chris M. Hostetter
• Created: 15/Sep/15 18:12
• Updated: 09/May/16 18:55
• Resolved: 11/Nov/15 17:49

Description

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)

Issue Links

• relates to: LUCENE-6789 change IndexSearcher default similarity to BM25
• relates to: SOLR-8271 use SchemaSimilarityFactory as default when no explicit (top level) sim is configured

Sub-Tasks

• Deprecate/rename DefaultSimilarityFactory to ClassicSimilarityFactory and remove DefaultSimilarityFactory in trunk: Closed
• Change the wrapped per-field default in SchemaSimilarityFactory to BM25 (conditional on luceneMatchVersion for backcompat): Resolved
• Make BM25SimFactory the implicit default when no sim is configured for luceneMatch > 6.0: Resolved