Always use constant_score query for match_only_text

CHANGELOG.md

@@ -86,6 +86,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Skip remote-repositories validations for node-joins when RepositoriesService is not in sync with cluster-state ([#16763](https://github.com/opensearch-project/OpenSearch/pull/16763))
 - Fix _list/shards API failing when closed indices are present ([#16606](https://github.com/opensearch-project/OpenSearch/pull/16606))
 - Fix remote shards balance ([#15335](https://github.com/opensearch-project/OpenSearch/pull/15335))
+- Always use `constant_score` query for `match_only_text` field ([#16964](https://github.com/opensearch-project/OpenSearch/pull/16964))
 
 ### Security
 

server/src/main/java/org/opensearch/index/mapper/MatchOnlyTextFieldMapper.java

@@ -16,6 +16,7 @@
 import org.apache.lucene.index.Term;
 import org.apache.lucene.search.BooleanClause;
 import org.apache.lucene.search.BooleanQuery;
+import org.apache.lucene.search.ConstantScoreQuery;
 import org.apache.lucene.search.MultiPhraseQuery;
 import org.apache.lucene.search.PhraseQuery;
 import org.apache.lucene.search.Query;
@@ -290,6 +291,16 @@ public Query phrasePrefixQuery(TokenStream stream, int slop, int maxExpansions,
             return new SourceFieldMatchQuery(builder.build(), phrasePrefixQuery, this, context);
         }
 
+        @Override
+        public Query termQuery(Object value, QueryShardContext context) {
+            return new ConstantScoreQuery(super.termQuery(value, context));
+        }
+
+        @Override
+        public Query termQueryCaseInsensitive(Object value, QueryShardContext context) {
+            return new ConstantScoreQuery(super.termQueryCaseInsensitive(value, context));
+        }
+
         private List<List<Term>> getTermsFromTokenStream(TokenStream stream) throws IOException {
             final List<List<Term>> termArray = new ArrayList<>();
             TermToBytesRefAttribute termAtt = stream.getAttribute(TermToBytesRefAttribute.class);

server/src/test/java/org/opensearch/index/mapper/MatchOnlyTextFieldMapperTests.java

@@ -15,11 +15,13 @@
 import org.apache.lucene.index.Term;
 import org.apache.lucene.search.BooleanClause;
 import org.apache.lucene.search.BooleanQuery;
+import org.apache.lucene.search.ConstantScoreQuery;
 import org.apache.lucene.search.MultiPhraseQuery;
 import org.apache.lucene.search.PhraseQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.TermQuery;
 import org.apache.lucene.tests.analysis.MockSynonymAnalyzer;
+import org.opensearch.common.lucene.search.AutomatonQueries;
 import org.opensearch.common.lucene.search.MultiPhrasePrefixQuery;
 import org.opensearch.core.common.Strings;
 import org.opensearch.core.xcontent.MediaTypeRegistry;
@@ -28,6 +30,7 @@
 import org.opensearch.index.query.MatchPhraseQueryBuilder;
 import org.opensearch.index.query.QueryShardContext;
 import org.opensearch.index.query.SourceFieldMatchQuery;
+import org.opensearch.index.query.TermQueryBuilder;
 import org.opensearch.index.search.MatchQuery;
 import org.junit.Before;
 
@@ -391,7 +394,7 @@ public void testPhraseQuery() throws IOException {
 
         assertThat(q, is(expectedQuery));
         Query q4 = new MatchPhraseQueryBuilder("field", "singleton").toQuery(queryShardContext);
-        assertThat(q4, is(new TermQuery(new Term("field", "singleton"))));
+        assertThat(q4, is(new ConstantScoreQuery(new TermQuery(new Term("field", "singleton")))));
 
         Query q2 = new MatchPhraseQueryBuilder("field", "three words here").toQuery(queryShardContext);
         expectedQuery = new SourceFieldMatchQuery(
@@ -447,4 +450,22 @@ public void testPhraseQuery() throws IOException {
         );
         assertThat(q6, is(expectedQuery));
     }
+
+    public void testTermQuery() throws Exception {
+        MapperService mapperService = createMapperService(mapping(b -> {
+            b.startObject("field");
+            {
+                b.field("type", textFieldName);
+                b.field("analyzer", "my_stop_analyzer"); // "standard" will be replaced with MockSynonymAnalyzer
+            }
+            b.endObject();
+        }));
+        QueryShardContext queryShardContext = createQueryShardContext(mapperService);
+
+        Query q = new TermQueryBuilder("field", "foo").rewrite(queryShardContext).toQuery(queryShardContext);
+        assertEquals(new ConstantScoreQuery(new TermQuery(new Term("field", "foo"))), q);
+
+        q = new TermQueryBuilder("field", "foo").caseInsensitive(true).rewrite(queryShardContext).toQuery(queryShardContext);
+        assertEquals(new ConstantScoreQuery(AutomatonQueries.caseInsensitiveTermQuery(new Term("field", "foo"))), q);
+    }
 }

server/src/test/java/org/opensearch/index/mapper/MatchOnlyTextFieldTypeTests.java

@@ -8,7 +8,11 @@
 
 package org.opensearch.index.mapper;
 
+import org.apache.lucene.index.Term;
+import org.apache.lucene.search.ConstantScoreQuery;
+import org.apache.lucene.search.TermQuery;
 import org.opensearch.common.lucene.Lucene;
+import org.opensearch.common.lucene.search.AutomatonQueries;
 
 public class MatchOnlyTextFieldTypeTests extends TextFieldTypeTests {
 
@@ -28,4 +32,18 @@ TextFieldMapper.TextFieldType createFieldType(boolean searchable) {
             ParametrizedFieldMapper.Parameter.metaParam().get()
         );
     }
+
+    @Override
+    public void testTermQuery() {
+        MappedFieldType ft = createFieldType(true);
+        assertEquals(new ConstantScoreQuery(new TermQuery(new Term("field", "foo"))), ft.termQuery("foo", null));
+        assertEquals(
+            new ConstantScoreQuery(AutomatonQueries.caseInsensitiveTermQuery(new Term("field", "fOo"))),
+            ft.termQueryCaseInsensitive("fOo", null)
+        );
+
+        MappedFieldType unsearchable = createFieldType(false);
+        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> unsearchable.termQuery("bar", null));
+        assertEquals("Cannot search on field [field] since it is not indexed.", e.getMessage());
+    }
 }