11.7. Batch fetching

<div class="paragraph">

Hibernate offers the [`@BatchSize`](https://docs.jboss.org/hibernate/orm/5.2/javadocs/org/hibernate/annotations/BatchSize.html) annotation,
which can be used when fetching uninitialized entity proxies.

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<div class="paragraph">

Considering the following entity mapping:

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<div id="fetching-batch-mapping-example" class="exampleblock">
<div class="title">Example 290. `@BatchSize` mapping example</div>
<div class="content">
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight">`@Entity(name = "Department")
public static class Department {

    @Id
    private Long id;

    @OneToMany(mappedBy = "department")
    //@BatchSize(size = 5)
    private List&lt;Employee&gt; employees = new ArrayList&lt;&gt;();

    //Getters and setters omitted for brevity

}

@Entity(name = "Employee")
public static class Employee {

    @Id
    private Long id;

    @NaturalId
    private String name;

    @ManyToOne(fetch = FetchType.LAZY)
    private Department department;

    //Getters and setters omitted for brevity
}`</pre>
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<div class="paragraph">

Considering that we have previously fetched several `Department` entities,
and now we need to initialize the `employees` entity collection for each particular `Department`,
the `@BatchSize` annotations allows us to load multiple `Employee` entities in a single database roundtrip.

</div>
<div id="fetching-batch-fetching-example" class="exampleblock">
<div class="title">Example 291. `@BatchSize` fetching example</div>
<div class="content">
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight">`List&lt;Department&gt; departments = entityManager.createQuery(
    "select d " +
    "from Department d " +
    "inner join d.employees e " +
    "where e.name like 'John%'", Department.class)
.getResultList();

for ( Department department : departments ) {
    log.infof(
        "Department %d has {} employees",
        department.getId(),
        department.getEmployees().size()
    );
}`</pre>
</div>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight">`SELECT
    d.id as id1_0_
FROM
    Department d
INNER JOIN
    Employee employees1_
    ON d.id=employees1_.department_id

SELECT
    e.department_id as departme3_1_1_,
    e.id as id1_1_1_,
    e.id as id1_1_0_,
    e.department_id as departme3_1_0_,
    e.name as name2_1_0_
FROM
    Employee e
WHERE
    e.department_id IN (
        0, 2, 3, 4, 5
    )

SELECT
    e.department_id as departme3_1_1_,
    e.id as id1_1_1_,
    e.id as id1_1_0_,
    e.department_id as departme3_1_0_,
    e.name as name2_1_0_
FROM
    Employee e
WHERE
    e.department_id IN (
        6, 7, 8, 9, 1
    )`</pre>
</div>
</div>
</div>
</div>
<div class="paragraph">

As you can see in the example above, there are only two SQL statements used to fetch the `Employee` entities associated to multiple `Department` entities.

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<td class="icon">

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<td class="content">
<div class="paragraph">

Without `@BatchSize`, you&#8217;d run into a N+1 query issue, so,
instead of 2 SQL statements, there would be 10 queries needed for fetching the `Employee` child entities.

</div>
<div class="paragraph">

However, although `@BatchSize` is better than running into an N+1 query issue,
most of the time, a DTO projection or a `JOIN FETCH` is a much better alternative since
it allows you to fetch all the required data with a single query.

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<div class="sect2">