HHH-11928 : Empty left join fetched collection is uninitialized when collection key is composite with hibernate.create_empty_composites.enabled=true

hibernate-core/src/main/java/org/hibernate/persister/collection/AbstractCollectionPersister.java

@@ -870,7 +870,16 @@ public abstract class AbstractCollectionPersister
 	@Override
 	public Object readKey(ResultSet rs, String[] aliases, SharedSessionContractImplementor session)
 			throws HibernateException, SQLException {
-		return getKeyType().nullSafeGet( rs, aliases, session, null );
+		// First hydrate the collection key to check if it is null.
+		// Don't bother resolving the collection key if the hydrated value is null.
+
+		// Implementation note: if collection key is a composite value, then resolving a null value will
+		// result in instantiating an empty composite if AvailableSettings#CREATE_EMPTY_COMPOSITES_ENABLED
+		// is true. By not resolving a null value for a composite key, we avoid the overhead of instantiating
+		// an empty composite, checking if it is equivalent to null (it should be), then ultimately throwing
+		// out the empty value.
+		final Object hydratedKey = getKeyType().hydrate( rs, aliases, session, null );
+		return hydratedKey == null ? null : getKeyType().resolve( hydratedKey, session, null );
 	}
 
 	/**