SOLR-13025: SchemaSimilarityFactory fallback to LegacyBM25Similarity for luceneMatchVersion < 8.0

solr/CHANGES.txt

@@ -59,6 +59,13 @@ Upgrade Notes
 * SOLR-12754: The UnifiedHighlighter hl.weightMatches now defaults to true.  If there are unforseen highlight problems,
   this may be the culprit.
 
+* If you explicitly use BM25SimilarityFactory in your schema, the absolute scoring will be lower due to SOLR-13025.
+  But ordering of documents will not change in the normal case. Use LegacyBM25SimilarityFactory if you need to force
+  the old 6.x/7.x scoring. Note that if you have not specified any similarity in schema or use the default 
+  SchemaSimilarityFactory, then LegacyBM25Similarity is automatically selected for 'luceneMatchVersion' < 8.0.0.
+  See also explanation in Reference Guide chapter "Other Schema Elements".
+
+
 New Features
 ----------------------
 
@@ -94,6 +101,10 @@ Optimizations
 
 * SOLR-12725: ParseDateFieldUpdateProcessorFactory should reuse ParsePosition. (ab)
 
+* SOLR-13025: Due to LUCENE-8563, the BM25Similarity formula no longer includes the (k1+1) factor in the numerator
+  This gives a lower absolute score but doesn't affect ordering, as this is a constant factor which is the same 
+  for every document. Use LegacyBM25SimilarityFactory if you need the old 6.x/7.x scoring. See also upgrade notes (janhoy)
+
 Other Changes
 ----------------------
 

solr/contrib/ltr/src/test/org/apache/solr/ltr/feature/TestExternalFeatures.java

@@ -70,7 +70,7 @@ public class TestExternalFeatures extends TestRerankBase {
     query.add("rq", "{!ltr reRankDocs=10 model=externalmodel efi.user_query=w3 efi.userTitlePhrase1=w4 efi.userTitlePhrase2=w5}");
 
     assertJQ("/query" + query.toQueryString(), "/response/docs/[0]/id=='3'");
-    assertJQ("/query" + query.toQueryString(), "/response/docs/[0]/score==0.7693934");
+    assertJQ("/query" + query.toQueryString(), "/response/docs/[0]/score==0.34972426");
     assertJQ("/query" + query.toQueryString(), "/response/docs/[1]/score==0.0");
     assertJQ("/query" + query.toQueryString(), "/response/docs/[2]/score==0.0");
 
@@ -80,7 +80,7 @@ public class TestExternalFeatures extends TestRerankBase {
     query.add("fl", "*,score,[fv efi.user_query=w2 efi.userTitlePhrase1=w4 efi.userTitlePhrase2=w5]");
 
     assertJQ("/query" + query.toQueryString(), "/response/docs/[0]/id=='3'");
-    assertJQ("/query" + query.toQueryString(), "/response/docs/[0]/score==0.7693934");
+    assertJQ("/query" + query.toQueryString(), "/response/docs/[0]/score==0.34972426");
     assertJQ("/query" + query.toQueryString(), "/response/docs/[1]/score==0.0");
     assertJQ("/query" + query.toQueryString(), "/response/docs/[2]/score==0.0");
   }

solr/core/src/java/org/apache/solr/search/similarities/BM25SimilarityFactory.java

@@ -16,13 +16,15 @@
  */
 package org.apache.solr.search.similarities;
 
+import org.apache.lucene.search.similarities.BM25Similarity;
 import org.apache.lucene.search.similarities.Similarity;
-import org.apache.lucene.search.similarity.LegacyBM25Similarity;
 import org.apache.solr.common.params.SolrParams;
 import org.apache.solr.schema.SimilarityFactory;
 
 /**
- * Factory for {@link LegacyBM25Similarity}
+ * Factory for BM25Similarity. This is the default similarity since 8.x.
+ * If you need the exact same formula as in 6.x and 7.x you should instead look at
+ * {@link LegacyBM25SimilarityFactory}
  * <p>
  * Parameters:
  * <ul>
@@ -35,9 +37,10 @@ import org.apache.solr.schema.SimilarityFactory;
  * Optional settings:
  * <ul>
  *   <li>discountOverlaps (bool): Sets
- *       {@link LegacyBM25Similarity#setDiscountOverlaps(boolean)}</li>
+ *       {@link BM25Similarity#setDiscountOverlaps(boolean)}</li>
  * </ul>
  * @lucene.experimental
+ * @since 8.0.0
  */
 public class BM25SimilarityFactory extends SimilarityFactory {
   private boolean discountOverlaps;
@@ -54,7 +57,7 @@ public class BM25SimilarityFactory extends SimilarityFactory {
 
   @Override
   public Similarity getSimilarity() {
-    LegacyBM25Similarity sim = new LegacyBM25Similarity(k1, b);
+    BM25Similarity sim = new BM25Similarity(k1, b);
     sim.setDiscountOverlaps(discountOverlaps);
     return sim;
   }

solr/core/src/java/org/apache/solr/search/similarities/LegacyBM25SimilarityFactory.java

@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.solr.search.similarities;
+
+import org.apache.lucene.search.similarities.Similarity;
+import org.apache.lucene.search.similarity.LegacyBM25Similarity;
+import org.apache.solr.common.params.SolrParams;
+import org.apache.solr.schema.SimilarityFactory;
+
+/**
+ * Factory for {@link LegacyBM25Similarity}. 
+ * Use this to force explicit creation of the BM25 formula that was used by BM25Similarity before Solr/Lucene 8.0.0.
+ * Note that {@link SchemaSimilarityFactory} will automatically create an instance of LegacyBM25Similarity if luceneMatchVersion is < 8.0.0
+ * <p>
+ * Parameters:
+ * <ul>
+ *   <li>k1 (float): Controls non-linear term frequency normalization (saturation).
+ *                   The default is <code>1.2</code>
+ *   <li>b (float): Controls to what degree document length normalizes tf values.
+ *                  The default is <code>0.75</code>
+ * </ul>
+ * <p>
+ * Optional settings:
+ * <ul>
+ *   <li>discountOverlaps (bool): Sets
+ *       {@link LegacyBM25Similarity#setDiscountOverlaps(boolean)}</li>
+ * </ul>
+ * @lucene.experimental
+ * @since 8.0.0
+ */
+public class LegacyBM25SimilarityFactory extends SimilarityFactory {
+  private boolean discountOverlaps;
+  private float k1;
+  private float b;
+
+  @Override
+  public void init(SolrParams params) {
+    super.init(params);
+    discountOverlaps = params.getBool("discountOverlaps", true);
+    k1 = params.getFloat("k1", 1.2f);
+    b = params.getFloat("b", 0.75f);
+  }
+
+  @Override
+  public Similarity getSimilarity() {
+    LegacyBM25Similarity sim = new LegacyBM25Similarity(k1, b);
+    sim.setDiscountOverlaps(discountOverlaps);
+    return sim;
+  }
+}

solr/core/src/java/org/apache/solr/search/similarities/SchemaSimilarityFactory.java

@@ -16,7 +16,7 @@
  */
 package org.apache.solr.search.similarities;
 
-import org.apache.lucene.search.similarities.ClassicSimilarity;
+import org.apache.lucene.search.similarities.BM25Similarity;
 import org.apache.lucene.search.similarities.PerFieldSimilarityWrapper;
 import org.apache.lucene.search.similarities.Similarity;
 import org.apache.lucene.search.similarity.LegacyBM25Similarity;
@@ -39,8 +39,8 @@ import org.apache.solr.util.plugin.SolrCoreAware;
  * matching configured:
  * </p>
  * <ul>
- *  <li><code>luceneMatchVersion &lt; 6.0</code> = {@link ClassicSimilarity}</li>
+ *  <li><code>luceneMatchVersion &lt; 8.0</code> = {@link LegacyBM25Similarity}</li>
- *  <li><code>luceneMatchVersion &gt;= 6.0</code> = {@link LegacyBM25Similarity}</li>
+ *  <li><code>luceneMatchVersion &gt;= 8.0</code> = {@link BM25Similarity}</li>
  * </ul>
  * <p>
  * The <code>defaultSimFromFieldType</code> option accepts the name of any fieldtype, and uses 
@@ -85,10 +85,12 @@ public class SchemaSimilarityFactory extends SimilarityFactory implements SolrCo
   
   private volatile SolrCore core; // set by inform(SolrCore)
   private volatile Similarity similarity; // lazy instantiated
+  private Version coreVersion = Version.LATEST;
 
   @Override
   public void inform(SolrCore core) {
     this.core = core;
+    this.coreVersion = this.core.getSolrConfig().luceneMatchVersion;
   }
   
   @Override
@@ -109,7 +111,9 @@ public class SchemaSimilarityFactory extends SimilarityFactory implements SolrCo
       Similarity defaultSim = null;
       if (null == defaultSimFromFieldType) {
         // nothing configured, choose a sensible implicit default...
-        defaultSim = new LegacyBM25Similarity();
+        defaultSim = coreVersion.onOrAfter(Version.LUCENE_8_0_0) ? 
+            new BM25Similarity() :
+            new LegacyBM25Similarity();
       } else {
         FieldType defSimFT = core.getLatestSchema().getFieldTypeByName(defaultSimFromFieldType);
         if (null == defSimFT) {

solr/core/src/test-files/solr/collection1/conf/schema-bm25.xml

@@ -36,10 +36,27 @@
     </similarity>
   </fieldType>
 
+  <!-- legacybm25 with default parameters -->
+  <fieldType name="legacy_text" class="solr.TextField">
+    <analyzer class="org.apache.lucene.analysis.standard.StandardAnalyzer"/>
+    <similarity class="solr.LegacyBM25SimilarityFactory"/>
+  </fieldType>
+
+  <!-- legacybm25 with parameters -->
+  <fieldType name="legacy_text_params" class="solr.TextField">
+    <analyzer class="org.apache.lucene.analysis.standard.StandardAnalyzer"/>
+    <similarity class="solr.LegacyBM25SimilarityFactory">
+      <float name="k1">1.2</float>
+      <float name="b">0.76</float>
+    </similarity>
+  </fieldType>
+  
 
   <field name="id" type="string" indexed="true" stored="true" multiValued="false" required="false"/>
   <field name="text" type="text" indexed="true" stored="false"/>
   <field name="text_params" type="text_params" indexed="true" stored="false"/>
+  <field name="legacy_text" type="legacy_text" indexed="true" stored="false"/>
+  <field name="legacy_text_params" type="legacy_text_params" indexed="true" stored="false"/>
 
   <uniqueKey>id</uniqueKey>
 

solr/core/src/test/org/apache/solr/rest/schema/TestBulkSchemaAPI.java

@@ -29,10 +29,10 @@ import java.util.function.Consumer;
 
 import org.apache.commons.io.FileUtils;
 import org.apache.lucene.misc.SweetSpotSimilarity;
+import org.apache.lucene.search.similarities.BM25Similarity;
 import org.apache.lucene.search.similarities.DFISimilarity;
 import org.apache.lucene.search.similarities.PerFieldSimilarityWrapper;
 import org.apache.lucene.search.similarities.Similarity;
-import org.apache.lucene.search.similarity.LegacyBM25Similarity;
 import org.apache.solr.common.SolrDocumentList;
 import org.apache.solr.core.CoreContainer;
 import org.apache.solr.core.SolrCore;
@@ -588,7 +588,7 @@ public class TestBulkSchemaAPI extends RestTestBase {
     assertNotNull("field a5 not created", m);
     assertEquals("myWhitespaceTxtField", m.get("type"));
     assertNull(m.get("uninvertible")); // inherited, but API shouldn't return w/o explicit showDefaults
-    assertFieldSimilarity("a5", LegacyBM25Similarity.class); // unspecified, expect default
+    assertFieldSimilarity("a5", BM25Similarity.class); // unspecified, expect default
 
     m = getObj(harness, "wdf_nocase", "fields");
     assertNull("field 'wdf_nocase' not deleted", m);
@@ -930,7 +930,7 @@ public class TestBulkSchemaAPI extends RestTestBase {
     Map fields = getObj(harness, fieldName, "fields");
     assertNotNull("field " + fieldName + " not created", fields);
     
-    assertFieldSimilarity(fieldName, LegacyBM25Similarity.class,
+    assertFieldSimilarity(fieldName, BM25Similarity.class,
        sim -> assertEquals("Unexpected k1", k1, sim.getK1(), .001),
        sim -> assertEquals("Unexpected b", b, sim.getB(), .001));
 

solr/core/src/test/org/apache/solr/search/TestPayloadScoreQParserPlugin.java

@@ -57,6 +57,6 @@ public class TestPayloadScoreQParserPlugin extends SolrTestCaseJ4 {
 
     // TODO: fix this includeSpanScore test to be less brittle - score result is score of "A" (via BM25) multipled by 1.0 (payload value)
     assertQ(req("fl","*,score", "q", "{!payload_score f=vals_dpf v=A func=min}"), "//float[@name='score']='1.0'");
-    assertQ(req("fl","*,score", "q", "{!payload_score f=vals_dpf v=A func=min includeSpanScore=true}"), "//float[@name='score']='0.2876821'");
+    assertQ(req("fl","*,score", "q", "{!payload_score f=vals_dpf v=A func=min includeSpanScore=true}"), "//float[@name='score']='0.13076457'");
   }
 }

solr/core/src/test/org/apache/solr/search/function/TestFunctionQuery.java

@@ -389,16 +389,16 @@ public class TestFunctionQuery extends SolrTestCaseJ4 {
     // superman has a higher df (thus lower idf) in one segment, but reversed in the complete index
     String q ="{!func}query($qq)";
     String fq="id:120"; 
-    assertQ(req("fl","*,score","q", q, "qq","text:batman", "fq",fq), "//float[@name='score']<'1.0'");
+    assertQ(req("fl","*,score","q", q, "qq","text:batman", "fq",fq), "//float[@name='score']<'0.6'");
-    assertQ(req("fl","*,score","q", q, "qq","text:superman", "fq",fq), "//float[@name='score']>'1.0'");
+    assertQ(req("fl","*,score","q", q, "qq","text:superman", "fq",fq), "//float[@name='score']>'0.6'");
 
     // test weighting through a function range query
-    assertQ(req("fl","*,score", "fq",fq,  "q", "{!frange l=1 u=10}query($qq)", "qq","text:superman"), "//*[@numFound='1']");
+    assertQ(req("fl","*,score", "fq",fq,  "q", "{!frange l=0.6 u=10}query($qq)", "qq","text:superman"), "//*[@numFound='1']");
 
     // test weighting through a complex function
     q ="{!func}sub(div(sum(0.0,product(1,query($qq))),1),0)";
-    assertQ(req("fl","*,score","q", q, "qq","text:batman", "fq",fq), "//float[@name='score']<'1.0'");
+    assertQ(req("fl","*,score","q", q, "qq","text:batman", "fq",fq), "//float[@name='score']<'0.6'");
-    assertQ(req("fl","*,score","q", q, "qq","text:superman", "fq",fq), "//float[@name='score']>'1.0'");
+    assertQ(req("fl","*,score","q", q, "qq","text:superman", "fq",fq), "//float[@name='score']>'0.6'");
 
 
     // test full param dereferencing

solr/core/src/test/org/apache/solr/search/similarities/TestBM25SimilarityFactory.java

@@ -16,8 +16,8 @@
  */
 package org.apache.solr.search.similarities;
 
+import org.apache.lucene.search.similarities.BM25Similarity;
 import org.apache.lucene.search.similarities.Similarity;
-import org.apache.lucene.search.similarity.LegacyBM25Similarity;
 import org.junit.BeforeClass;
 
 /**
@@ -31,14 +31,14 @@ public class TestBM25SimilarityFactory extends BaseSimilarityTestCase {
   
   /** bm25 with default parameters */
   public void test() throws Exception {
-    assertEquals(LegacyBM25Similarity.class, getSimilarity("text").getClass());
+    assertEquals(BM25Similarity.class, getSimilarity("text").getClass());
   }
   
   /** bm25 with parameters */
   public void testParameters() throws Exception {
     Similarity sim = getSimilarity("text_params");
-    assertEquals(LegacyBM25Similarity.class, sim.getClass());
+    assertEquals(BM25Similarity.class, sim.getClass());
-    LegacyBM25Similarity bm25 = (LegacyBM25Similarity) sim;
+    BM25Similarity bm25 = (BM25Similarity) sim;
     assertEquals(1.2f, bm25.getK1(), 0.01f);
     assertEquals(0.76f, bm25.getB(), 0.01f);
   }

solr/core/src/test/org/apache/solr/search/similarities/TestLegacyBM25SimilarityFactory.java

@@ -0,0 +1,45 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.solr.search.similarities;
+
+import org.apache.lucene.search.similarities.Similarity;
+import org.apache.lucene.search.similarity.LegacyBM25Similarity;
+import org.junit.BeforeClass;
+
+/**
+ * Tests {@link LegacyBM25SimilarityFactory}
+ */
+public class TestLegacyBM25SimilarityFactory extends BaseSimilarityTestCase {
+  @BeforeClass
+  public static void beforeClass() throws Exception {
+    initCore("solrconfig-basic.xml","schema-bm25.xml");
+  }
+  
+  /** bm25 with default parameters */
+  public void test() throws Exception {
+    assertEquals(LegacyBM25Similarity.class, getSimilarity("legacy_text").getClass());
+  }
+  
+  /** bm25 with parameters */
+  public void testParameters() throws Exception {
+    Similarity sim = getSimilarity("legacy_text_params");
+    assertEquals(LegacyBM25Similarity.class, sim.getClass());
+    LegacyBM25Similarity bm25 = (LegacyBM25Similarity) sim;
+    assertEquals(1.2f, bm25.getK1(), 0.01f);
+    assertEquals(0.76f, bm25.getB(), 0.01f);
+  }
+}

solr/core/src/test/org/apache/solr/search/similarities/TestNonDefinedSimilarityFactory.java

@@ -16,15 +16,19 @@
  */
 package org.apache.solr.search.similarities;
 
+import org.apache.lucene.search.similarities.BM25Similarity;
 import org.apache.lucene.search.similarity.LegacyBM25Similarity;
+import org.apache.lucene.util.Version;
 import org.junit.After;
 
 /**
- * Verifies that the default behavior of the implicit {@link ClassicSimilarityFactory} 
+ * Verifies that the default behavior of the implicit {@link BM25Similarity} 
  * (ie: no similarity configured in schema.xml at all) is consistent with 
  * expectations based on the luceneMatchVersion
  * @see <a href="https://issues.apache.org/jira/browse/SOLR-5561">SOLR-5561</a>
  * @see <a href="https://issues.apache.org/jira/browse/SOLR-8057">SOLR-8057</a>
+ * @see <a href="https://issues.apache.org/jira/browse/SOLR-13025">SOLR-13025</a>
+ * @see <a href="https://issues.apache.org/jira/browse/LUCENE-8563">LUCENE-8563</a>
  */
 public class TestNonDefinedSimilarityFactory extends BaseSimilarityTestCase {
 
@@ -33,10 +37,30 @@ public class TestNonDefinedSimilarityFactory extends BaseSimilarityTestCase {
     deleteCore();
   }
 
-  public void testCurrentBM25() throws Exception {
+  public void testCurrentBM25FromV8() throws Exception {
     // no sys prop set, rely on LATEST
     initCore("solrconfig-basic.xml","schema-tiny.xml");
-    LegacyBM25Similarity sim = getSimilarity("text", LegacyBM25Similarity.class);
+    BM25Similarity sim = getSimilarity("text", BM25Similarity.class);
     assertEquals(0.75F, sim.getB(), 0.0F);
   }
+
+  public void testLegacyBM25BeforeV8() throws Exception {
+    System.setProperty("tests.luceneMatchVersion", Version.LUCENE_7_0_0.toString());
+    initCore("solrconfig-basic.xml","schema-tiny.xml");
+    System.clearProperty("tests.luceneMatchVersion");
+    LegacyBM25Similarity sim = getSimilarity("text", LegacyBM25Similarity.class);
+    assertEquals(0.75F, sim.getB(), 0.0F);
+    deleteCore();
+
+    System.setProperty("tests.luceneMatchVersion", "5.0.0");
+    initCore("solrconfig-basic.xml","schema-tiny.xml");
+    System.clearProperty("tests.luceneMatchVersion");
+    getSimilarity("text", LegacyBM25Similarity.class);
+    deleteCore();
+
+    System.setProperty("tests.luceneMatchVersion", "6.0.0");
+    initCore("solrconfig-basic.xml","schema-tiny.xml");
+    System.clearProperty("tests.luceneMatchVersion");
+    getSimilarity("text", LegacyBM25Similarity.class);
+  }
 }

solr/core/src/test/org/apache/solr/search/similarities/TestPerFieldSimilarity.java

@@ -17,8 +17,8 @@
 package org.apache.solr.search.similarities;
 
 import org.apache.lucene.misc.SweetSpotSimilarity;
+import org.apache.lucene.search.similarities.BM25Similarity;
 import org.apache.lucene.search.similarities.Similarity;
-import org.apache.lucene.search.similarity.LegacyBM25Similarity;
 import org.junit.BeforeClass;
 
 /**
@@ -58,18 +58,18 @@ public class TestPerFieldSimilarity extends BaseSimilarityTestCase {
   /** test a field where no similarity is specified */
   public void testDefaults() throws Exception {
     Similarity sim = getSimilarity("sim3text");
-    assertEquals(LegacyBM25Similarity.class, sim.getClass());;
+    assertEquals(BM25Similarity.class, sim.getClass());;
   }
   
   /** ... and for a dynamic field */
   public void testDefaultsDynamic() throws Exception {
     Similarity sim = getSimilarity("text_sim3");
-    assertEquals(LegacyBM25Similarity.class, sim.getClass());
+    assertEquals(BM25Similarity.class, sim.getClass());
   }
   
   /** test a field that does not exist */
   public void testNonexistent() throws Exception {
     Similarity sim = getSimilarity("sdfdsfdsfdswr5fsdfdsfdsfs");
-    assertEquals(LegacyBM25Similarity.class, sim.getClass());
+    assertEquals(BM25Similarity.class, sim.getClass());
   }
 }

solr/solr-ref-guide/src/major-changes-in-solr-8.adoc

@@ -23,3 +23,9 @@
 // *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 
 * Due to LIR redesign in SOLR-11702, all users must be on Solr 7.3 or higher to upgrade to Solr 8
+
+* If you explicitly use BM25SimilarityFactory in your schema the absolute scoring will be lower, see SOLR-13025.
+  But ordering of documents will not change in the normal case. Use LegacyBM25SimilarityFactory if you need to force
+  the old 6.x/7.x scoring. Note that if you have not specified any similarity in schema or use the default 
+  SchemaSimilarityFactory, then LegacyBM25Similarity is automatically selected for 'luceneMatchVersion' < 8.0.0.
+  See also explanation in Reference Guide chapter "Other Schema Elements".

solr/solr-ref-guide/src/other-schema-elements.adoc

@@ -90,6 +90,6 @@ In most cases, specifying global level similarity like this will cause an error
 
 In the example above `IBSimilarityFactory` (using the Information-Based model) will be used for any fields of type `text_ib`, while `DFRSimilarityFactory` (divergence from random) will be used for any fields of type `text_dfr`, as well as any fields using a type that does not explicitly specify a `<similarity/>`.
 
-If `SchemaSimilarityFactory` is explicitly declared without configuring a `defaultSimFromFieldType`, then `BM25Similarity` is implicitly used as the default.
+If `SchemaSimilarityFactory` is explicitly declared without configuring a `defaultSimFromFieldType`, then `BM25Similarity` is implicitly used as the default for `luceneMatchVersion >= 8.0.0` and otherwise `LegacyBM25Similarity` is used to mimic the same BM25 formula that was the default in those versions.
 
-In addition to the various factories mentioned on this page, there are several other similarity implementations that can be used such as the `SweetSpotSimilarityFactory`, `ClassicSimilarityFactory`, etc. For details, see the Solr Javadocs for the {solr-javadocs}/solr-core/org/apache/solr/schema/SimilarityFactory.html[similarity factories].
+In addition to the various factories mentioned on this page, there are several other similarity implementations that can be used such as the `SweetSpotSimilarityFactory`, `ClassicSimilarityFactory`, `LegacyBM25SimilarityFactory` etc. For details, see the Solr Javadocs for the {solr-javadocs}/solr-core/org/apache/solr/schema/SimilarityFactory.html[similarity factories].