[GitHub] [lucene-solr] jtibshirani commented on a change in pull request #2293: LUCENE-9725: Allow BM25FQuery to use other similarities.

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jtibshirani commented on a change in pull request #2293:
URL: https://github.com/apache/lucene-solr/pull/2293#discussion_r570517621



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File path: lucene/sandbox/src/java/org/apache/lucene/sandbox/search/CombinedFieldQuery.java
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@@ -64,39 +62,35 @@
  * A {@link Query} that treats multiple fields as a single stream and scores terms as if you had
  * indexed them as a single term in a single field.
  *
- * <p>For scoring purposes this query implements the BM25F's simple formula described in:
- * http://www.staff.city.ac.uk/~sb317/papers/foundations_bm25_review.pdf
+ * <p>The query works as follows:
  *
- * <p>The per-field similarity is ignored but to be compatible each field must use a {@link
- * Similarity} at index time that encodes norms the same way as {@link SimilarityBase#computeNorm}.
+ * <ol>
+ *   <li>Given a list of fields and weights, it pretends there is a synthetic combined field where
+ *       all terms have been indexed. It computes new term and collection statistics for this
+ *       combined field.
+ *   <li>It uses a disjunction iterator and {@link IndexSearcher#getSimilarity} to score documents.
+ * </ol>
+ *
+ * <p>In order for a similarity to be compatible, {@link Similarity#computeNorm} must be additive:
+ * the norm of the combined field is the sum of norms for each individual field. This is usually
+ * true, since norms often represent the field length. Per-field similarities are not supported.

Review comment:
       Good point! I mentioned those similarities and restored the reference to `SimilarityBase#computeNorm`, which gives a clear example of what's acceptable.




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