HHH-19915 Re-use existing initialized collection instance when reading from previous row

Author: mbellade

hibernate-core/src/main/java/org/hibernate/sql/results/graph/collection/internal/AbstractImmediateCollectionInitializer.java

@@ -26,6 +26,8 @@
 
 import org.checkerframework.checker.nullness.qual.Nullable;
 
+import static org.hibernate.internal.util.NullnessUtil.castNonNull;
+
 /**
  * Base support for CollectionInitializer implementations that represent
  * an immediate initialization of some sort (join, select, batch, sub-select)
@@ -141,7 +143,39 @@ public void resolveState(Data data) {
 public void resolveFromPreviousRow(Data data) {
 super.resolveFromPreviousRow( data );
 if ( data.getState() == State.RESOLVED ) {
-resolveKeySubInitializers( data );
+// The state is resolved, so we know a collection instance exists
+final PersistentCollection<?> collection = castNonNull( data.getCollectionInstance() );
+if ( collection.wasInitialized() ) {
+// The collection was an already initialized instance, so we can set this to initialized
+// and just resolve sub-initializers
+resolveFromPreviouslyInitializedInstance( data );
+}
+else {
+resolveKeySubInitializers( data );
+}
+}
+}
+
+protected void resolveFromPreviouslyInitializedInstance(Data data) {
+data.setState( State.INITIALIZED );
+if ( data.shallowCached ) {
+initializeShallowCached( data );
+}
+else {
+resolveInstanceSubInitializers( data );
+}
+final var rowProcessingState = data.getRowProcessingState();
+if ( rowProcessingState.needsResolveState() ) {
+// Resolve the state of the identifier if result caching is enabled and this is not a query cache hit
+if ( collectionKeyResultAssembler != null ) {
+collectionKeyResultAssembler.resolveState( rowProcessingState );
+}
+if ( !getInitializingCollectionDescriptor().useShallowQueryCacheLayout() ) {
+if ( collectionValueKeyResultAssembler != null ) {
+collectionValueKeyResultAssembler.resolveState( rowProcessingState );
+}
+resolveCollectionContentState( rowProcessingState );
+}
 }
 }
 
@@ -337,25 +371,7 @@ public void resolveInstance(Object instance, Data data) {
 }
 data.collectionValueKey = null;
 if ( collection.wasInitialized() ) {
-data.setState( State.INITIALIZED );
-if ( data.shallowCached ) {
-initializeShallowCached( data );
-}
-else {
-resolveInstanceSubInitializers( data );
-}
-if ( rowProcessingState.needsResolveState() ) {
-// Resolve the state of the identifier if result caching is enabled and this is not a query cache hit
-if ( collectionKeyResultAssembler != null ) {
-collectionKeyResultAssembler.resolveState( rowProcessingState );
-}
-if ( !getInitializingCollectionDescriptor().useShallowQueryCacheLayout() ) {
-if ( collectionValueKeyResultAssembler != null ) {
-collectionValueKeyResultAssembler.resolveState( rowProcessingState );
-}
-resolveCollectionContentState( rowProcessingState );
-}
-}
+resolveFromPreviouslyInitializedInstance( data );
 }
 else {
 if ( data.shallowCached ) {

hibernate-core/src/test/java/org/hibernate/orm/test/bytecode/enhancement/detached/collection/DetachedCollectionInitializationJoinFetchTest.java

@@ -69,6 +69,14 @@ public void testInitializedDetachedInstance(SessionFactoryScope scope) {
 } );
 }
 
+@Test
+public void testExistingPersistentInstance(SessionFactoryScope scope) {
+scope.inTransaction( session -> {
+final var entityA = session.find( EntityA.class, 1L );
+fetchQuery( new ArrayList<>( entityA.getB() ), session );
+} );
+}
+
 @BeforeEach
 public void setUp(SessionFactoryScope scope) {
 scope.inTransaction( session -> {

hibernate-core/src/test/java/org/hibernate/orm/test/bytecode/enhancement/detached/collection/DetachedNonJoinedCollectionInitializationJoinFetchTest.java

@@ -71,6 +71,14 @@ public void testInitializedDetachedInstance(SessionFactoryScope scope) {
 } );
 }
 
+@Test
+public void testExistingPersistentInstance(SessionFactoryScope scope) {
+scope.inTransaction( session -> {
+final var entityA = session.find( EntityA.class, 1L );
+fetchQuery( new ArrayList<>( entityA.getB() ), session );
+} );
+}
+
 @BeforeEach
 public void setUp(SessionFactoryScope scope) {
 scope.inTransaction( session -> {