Query.getResultStream() throws OOM

Hibernate ORM
1

I am trying to refactor some JPA code and make it more readable by using javax.persistence.TypedQuery#getResultStream (source code: https://github.com/behrangsa/hibernate-get-result-stream).

I thought returning a Stream instead of a List will prevent loading the entire data set into memory and will have a lower memory footprint. However in my tests I am getting OutOfMemoryError:

persistence.xml

<persistence xmlns="http://java.sun.com/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
             version="2.0">
    <persistence-unit name="hibstr">
        <description>Hibernate EntityManager Demo</description>
        <class>org.behrang.misc.hibstr.Post</class>
        <properties>
            <property name="hibernate.dialect" value="org.hibernate.dialect.PostgreSQL95Dialect"/>
            <property name="hibernate.hbm2ddl.auto" value="validate"/>
            <property name="javax.persistence.jdbc.driver" value="org.postgresql.Driver"/>
            <property name="javax.persistence.jdbc.url"
                      value="jdbc:postgresql://localhost:32768/benchmark?currentSchema=benchmark"/>
            <property name="javax.persistence.jdbc.user" value="postgres"/>
            <property name="javax.persistence.jdbc.password" value="password"/>
        </properties>
    </persistence-unit>
</persistence>

PostTest

package org.behrang.misc.hibstr;

import org.hibernate.jpa.QueryHints;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.TypedQuery;
import java.util.stream.Stream;

public class PostTest {

    private static EntityManagerFactory entityManagerFactory;

    @BeforeClass
    public static void setup() {
        entityManagerFactory = Persistence.createEntityManagerFactory("hibstr");
    }

    @AfterClass
    public static void teardown() {
        if (entityManagerFactory.isOpen()) {
            entityManagerFactory.close();
        }
    }

    @Test
    public void shouldNotThrowOutOfMemoryError() {
        final EntityManager entityManager = entityManagerFactory.createEntityManager();
        entityManager.getTransaction().begin();

        final TypedQuery<Post> query = entityManager.createQuery("SELECT p from Post p", Post.class);
        query.setHint(QueryHints.HINT_FETCH_SIZE, 25);
        query.setHint(QueryHints.HINT_READONLY, "true");

        try (final Stream<Post> posts = query.getResultStream()) {
            posts.forEach(p -> {
                System.out.println(p.getId());
            });
        }

        entityManager.getTransaction().commit();
        entityManager.close();
    }

}

This outputs:

1
2
3
...
6362

Java heap space
java.lang.OutOfMemoryError: Java heap space

Any ideas what am I missing here?

EDIT: MySQL also causes OOM

See: https://github.com/behrangsa/hibernate-get-result-stream/tree/mysql