From 820fe56aa46d5367122c7cca35a13608da63cee2 Mon Sep 17 00:00:00 2001
From: Sanne Grinovero <sanne@hibernate.org>
Date: Thu, 29 Oct 2020 09:36:53 +0000
Subject: [PATCH] HHH-14305 Memory savings in SingleTableEntityPersister

---
 .../internal/AbstractIdentifiableType.java    | 25 +++++++++++++------
 .../entity/SingleTableEntityPersister.java    |  3 +++
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/hibernate-core/src/main/java/org/hibernate/metamodel/model/domain/internal/AbstractIdentifiableType.java b/hibernate-core/src/main/java/org/hibernate/metamodel/model/domain/internal/AbstractIdentifiableType.java
index ed7b041701..54a93363c6 100644
--- a/hibernate-core/src/main/java/org/hibernate/metamodel/model/domain/internal/AbstractIdentifiableType.java
+++ b/hibernate-core/src/main/java/org/hibernate/metamodel/model/domain/internal/AbstractIdentifiableType.java
@@ -7,6 +7,7 @@
 package org.hibernate.metamodel.model.domain.internal;
 
 import java.io.Serializable;
+import java.util.Collections;
 import java.util.HashSet;
 import java.util.Set;
 import java.util.function.Consumer;
@@ -207,6 +208,11 @@ public abstract class AbstractIdentifiableType<J>
 	@SuppressWarnings("unchecked")
 	public void collectIdClassAttributes(Set<SingularPersistentAttribute<? super J,?>> attributes) {
 		if ( idClassAttributes != null ) {
+			//Make it writeable, if it was using the EMPTY_SET marker.
+			// This is because there's a semantic difference in this class between a null and an empty Set.
+			if ( idClassAttributes == Collections.EMPTY_SET ) {
+				idClassAttributes = new HashSet<>();
+			}
 			attributes.addAll( idClassAttributes );
 		}
 		else if ( getSuperType() != null ) {
@@ -305,14 +311,19 @@ public abstract class AbstractIdentifiableType<J>
 
 		@Override
 		public void applyIdClassAttributes(Set<SingularPersistentAttribute<? super J,?>> idClassAttributes) {
-			for ( SingularAttribute<? super J,?> idClassAttribute : idClassAttributes ) {
-				if ( AbstractIdentifiableType.this == idClassAttribute.getDeclaringType() ) {
-					@SuppressWarnings({ "unchecked" })
-					SingularPersistentAttribute<J,?> declaredAttribute = (SingularPersistentAttribute) idClassAttribute;
-					addAttribute( declaredAttribute );
-				}
+			if ( idClassAttributes.isEmpty() ) {
+				AbstractIdentifiableType.this.idClassAttributes = Collections.EMPTY_SET;
+			}
+			else {
+				for ( SingularAttribute<? super J,?> idClassAttribute : idClassAttributes ) {
+					if ( AbstractIdentifiableType.this == idClassAttribute.getDeclaringType() ) {
+						@SuppressWarnings({ "unchecked" })
+						SingularPersistentAttribute<J,?> declaredAttribute = (SingularPersistentAttribute) idClassAttribute;
+						addAttribute( declaredAttribute );
+					}
+				}
+				AbstractIdentifiableType.this.idClassAttributes = idClassAttributes;
 			}
-			AbstractIdentifiableType.this.idClassAttributes = idClassAttributes;
 		}
 
 		@Override
diff --git a/hibernate-core/src/main/java/org/hibernate/persister/entity/SingleTableEntityPersister.java b/hibernate-core/src/main/java/org/hibernate/persister/entity/SingleTableEntityPersister.java
index b7ad8a73c6..9086a449d2 100644
--- a/hibernate-core/src/main/java/org/hibernate/persister/entity/SingleTableEntityPersister.java
+++ b/hibernate-core/src/main/java/org/hibernate/persister/entity/SingleTableEntityPersister.java
@@ -8,6 +8,7 @@ package org.hibernate.persister.entity;
 
 import java.io.Serializable;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
@@ -277,6 +278,8 @@ public class SingleTableEntityPersister extends AbstractEntityPersister {
 		isNullableSubclassTable = ArrayHelper.toBooleanArray( isNullables );
 		hasSequentialSelects = hasDeferred;
 
+		this.sequentialSelectStringsByEntityName = hasSequentialSelects ? new HashMap<>() : Collections.EMPTY_MAP;
+
 		// DISCRIMINATOR
 
 		if ( persistentClass.isPolymorphic() ) {