Github user jpountz commented on a diff in the pull request:
https://github.com/apache/lucene-solr/pull/511#discussion_r237388295
--- Diff: lucene/MIGRATE.txt ---
@@ -150,3 +150,11 @@ in order to support ToParent/ToChildBlockJoinQuery.
Normalization is now type-safe, with CharFilterFactory#normalize()
returning a Reader and
TokenFilterFactory#normalize() returning a TokenFilter.
+
+## k1+1 constant factor removed from BM25 similarity numerator
+
+Scores computed by the BM25 similarity are lower than previously as the
k1+1
+constant factor was removed from the numerator of the scoring formula.
+Ordering of results is preserved unless scores are computed from multiple
+fields using different similarities. The previous behaviour is now exposed
+through the LegacyBM25Similarity class.
--- End diff --
maybe add that it can be found in the lucene-misc jar?
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