15.54. Read-only entities

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As explained in [entity immutability](#entity-immutability) section, fetching entities in read-only mode is much more efficient than fetching read-write entities.
Even if the entities are mutable, you can still fetch them in read-only mode, and benefit from reducing the memory footprint and sepeding up the flushing process.

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Read-only entities are skipped by the dirty checking mechanism as illustrated by the following example:

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<div id="hql-read-only-entities-example" class="exampleblock">
<div class="title">Example 407. Read-only entities query example</div>
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<pre class="prettyprint highlight">`List&lt;Call&gt; calls = entityManager.createQuery(
    "select c " +
    "from Call c " +
    "join c.phone p " +
    "where p.number = :phoneNumber ", Call.class )
.setParameter( "phoneNumber", "123-456-7890" )
.setHint( "org.hibernate.readOnly", true )
.getResultList();

calls.forEach( c -&gt; c.setDuration( 0 ) );`</pre>
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<pre class="prettyprint highlight">`SELECT c.id AS id1_5_ ,
       c.duration AS duration2_5_ ,
       c.phone_id AS phone_id4_5_ ,
       c.call_timestamp AS call_tim3_5_
FROM   phone_call c
INNER JOIN phone p ON c.phone_id = p.id
WHERE   p.phone_number = '123-456-7890'`</pre>
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As you can see, there is no SQL `UPDATE` being executed.

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The Hibernate native API offers a `Query#setReadOnly` method, as an alternative to using a JPA query hint:

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<div id="hql-read-only-entities-native-example" class="exampleblock">
<div class="title">Example 408. Read-only entities native query example</div>
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<div class="listingblock">
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<pre class="prettyprint highlight">`List&lt;Call&gt; calls = entityManager.createQuery(
    "select c " +
    "from Call c " +
    "join c.phone p " +
    "where p.number = :phoneNumber ", Call.class )
.setParameter( "phoneNumber", "123-456-7890" )
.unwrap( org.hibernate.query.Query.class )
.setReadOnly( true )
.getResultList();`</pre>
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