11.9. FetchMode.SELECT

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To demonstrate how `FetchMode.SELECT` works, consider the following entity mapping:

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<div id="fetching-strategies-fetch-mode-select-mapping-example" class="exampleblock">
<div class="title">Example 292. `FetchMode.SELECT` mapping example</div>
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<pre class="prettyprint highlight">`@Entity(name = "Department")
public static class Department {

    @Id
    private Long id;

    @OneToMany(mappedBy = "department", fetch = FetchType.LAZY)
    @Fetch(FetchMode.SELECT)
    private List&lt;Employee&gt; employees = new ArrayList&lt;&gt;();

    //Getters and setters omitted for brevity

}

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

    @Id
    @GeneratedValue
    private Long id;

    @NaturalId
    private String username;

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

    //Getters and setters omitted for brevity

}`</pre>
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Considering there are multiple `Department` entities, each one having multiple `Employee` entities,
when executing the following test case, Hibernate fetches every uninitialized `Employee`
collection using a secondary `SELECT` statement upon accessing the child collection for the first time:

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<div id="fetching-strategies-fetch-mode-select-example" class="exampleblock">
<div class="title">Example 293. `FetchMode.SELECT` mapping example</div>
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<pre class="prettyprint highlight">`List&lt;Department&gt; departments = entityManager.createQuery(
    "select d from Department d", Department.class )
.getResultList();

log.infof( "Fetched %d Departments", departments.size());

for (Department department : departments ) {
    assertEquals( 3, department.getEmployees().size() );
}`</pre>
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<div class="content">
<pre class="prettyprint highlight">`SELECT
    d.id as id1_0_
FROM
    Department d

-- Fetched 2 Departments

SELECT
    e.department_id as departme3_1_0_,
    e.id as id1_1_0_,
    e.id as id1_1_1_,
    e.department_id as departme3_1_1_,
    e.username as username2_1_1_
FROM
    Employee e
WHERE
    e.department_id = 1

SELECT
    e.department_id as departme3_1_0_,
    e.id as id1_1_0_,
    e.id as id1_1_1_,
    e.department_id as departme3_1_1_,
    e.username as username2_1_1_
FROM
    Employee e
WHERE
    e.department_id = 2`</pre>
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<div class="paragraph">

The more `Department` entities are fetched by the first query, the more secondary `SELECT` statements are executed to initialize the `employees` collections.
Therefore, `FetchMode.SELECT` can lead to N+1 query issues.

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