HHH-18835 Use multi-table insert also for generators that don't support bulk inserts

hibernate-core/src/main/java/org/hibernate/persister/entity/AbstractEntityPersister.java

@@ -4819,10 +4819,11 @@ private boolean isSubselect() {
 
 	private boolean generatorNeedsMultiTableInsert() {
 		final var generator = getGenerator();
-		if ( generator instanceof BulkInsertionCapableIdentifierGenerator
+		if ( generator instanceof BulkInsertionCapableIdentifierGenerator bulkInsertionCapableGenerator
 				&& generator instanceof OptimizableGenerator optimizableGenerator ) {
 			final var optimizer = optimizableGenerator.getOptimizer();
-			return optimizer != null && optimizer.getIncrementSize() > 1;
+			return optimizer != null && optimizer.getIncrementSize() > 1
+				|| !bulkInsertionCapableGenerator.supportsBulkInsertionIdentifierGeneration();
 		}
 		else {
 			return false;

hibernate-core/src/main/java/org/hibernate/query/sqm/internal/SqmQueryImpl.java

@@ -625,10 +625,11 @@ private boolean useMultiTableInsert(EntityPersister persister, SqmInsertStatemen
 		boolean useMultiTableInsert = persister.hasMultipleTables();
 		if ( !useMultiTableInsert && !isSimpleValuesInsert( sqmInsert, persister ) ) {
 			final var identifierGenerator = persister.getGenerator();
-			if ( identifierGenerator instanceof BulkInsertionCapableIdentifierGenerator
+			if ( identifierGenerator instanceof BulkInsertionCapableIdentifierGenerator bulkInsertionCapableGenerator
 					&& identifierGenerator instanceof OptimizableGenerator optimizableGenerator ) {
 				final var optimizer = optimizableGenerator.getOptimizer();
-				if ( optimizer != null && optimizer.getIncrementSize() > 1 ) {
+				if ( optimizer != null && optimizer.getIncrementSize() > 1
+					|| !bulkInsertionCapableGenerator.supportsBulkInsertionIdentifierGeneration() ) {
 					useMultiTableInsert = !hasIdentifierAssigned( sqmInsert, persister );
 				}
 			}

hibernate-core/src/test/java/org/hibernate/orm/test/query/hql/InsertSelectLegacyTest.java

@@ -0,0 +1,82 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ * Copyright Red Hat Inc. and Hibernate Authors
+ */
+package org.hibernate.orm.test.query.hql;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.GeneratedValue;
+import jakarta.persistence.Id;
+import jakarta.persistence.ManyToOne;
+import org.hibernate.cfg.AvailableSettings;
+import org.hibernate.id.enhanced.LegacyNamingStrategy;
+import org.hibernate.testing.orm.junit.DomainModel;
+import org.hibernate.testing.orm.junit.JiraKey;
+import org.hibernate.testing.orm.junit.ServiceRegistry;
+import org.hibernate.testing.orm.junit.SessionFactory;
+import org.hibernate.testing.orm.junit.SessionFactoryScope;
+import org.hibernate.testing.orm.junit.Setting;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+@DomainModel(
+		annotatedClasses = {
+				InsertSelectLegacyTest.Person.class,
+				InsertSelectLegacyTest.Document.class
+		}
+)
+@ServiceRegistry(
+		settings = {
+				@Setting(name = AvailableSettings.SHOW_SQL, value = "true"),
+				@Setting(name = AvailableSettings.FORMAT_SQL, value = "true"),
+				@Setting(name = AvailableSettings.ID_DB_STRUCTURE_NAMING_STRATEGY,
+						value = LegacyNamingStrategy.STRATEGY_NAME)
+		}
+)
+@SessionFactory
+public class InsertSelectLegacyTest {
+
+	@Test
+	@JiraKey(value = "HHH-18835")
+	void testInsertSelect(SessionFactoryScope scope) {
+
+		scope.inTransaction( session -> {
+			Person person = new Person();
+			person.name = "Peter";
+			session.persist( person );
+
+			session.createMutationQuery(
+							"insert into Document(name,owner) select concat(p.name,'s document'), p from Person p" )
+					.executeUpdate();
+
+			Document document = session.createQuery( "select d from Document d", Document.class ).getSingleResult();
+			assertNotNull( document );
+			assertEquals( "Peters document", document.name );
+		} );
+	}
+
+	@Entity(name = "Person")
+	public static class Person {
+
+		@Id
+		@GeneratedValue
+		Long id;
+
+		String name;
+	}
+
+	@Entity(name = "Document")
+	public static class Document {
+
+		@Id
+		@GeneratedValue
+		Long id;
+
+		String name;
+
+		@ManyToOne
+		Person owner;
+	}
+}