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<div class="sect1"><!-- infolevel="all" infotype="General" -->
<h1 class="sect1"><font face="arial, helvetica, sans-serif" color="#330099">About Query Hints</font></h1>
<p>You can use a query hint to customize or optimize a JPA query. The <code>NamedQuery</code> annotation is used to specify a named query in the Java Persistence query language. This annotation contains a <code>hints</code> element that can be used to specify query properties and hints. For more information on this annotation, see "NamedQuery Annotation" in the JPA Specification.</p>
<p><code><a href="http://jcp.org/en/jsr/detail?id=338">http://jcp.org/en/jsr/detail?id=338</a></code></p>
<p>The definitions of query hints are vendor-specific. The following sections describe JPA query hints and EclipseLink query hints:</p>
<ul>
<li>
<p><a href="#CIHFICDB">JPA Cache Query Hints</a></p>
</li>
<li>
<p><a href="#CIHJEHDG">EclipseLink Extensions to Cache Query Hints</a></p>
</li>
</ul>
<a id="CIHFICDB" name="CIHFICDB"></a>
<div class="sect2">
<h2 class="sect2"><font face="arial, helvetica, sans-serif" color="#330099">JPA Cache Query Hints</font></h2>
<p>The JPA query hints allow for queries or the <code>find()</code> operation to bypass, or refresh the shared cache. JPA cache query hints can be set on named or dynamic queries, or set in the properties map passed to the <code>find()</code> operation.</p>
<p>JPA 2.0 defines the following query hint properties to configure a queries interaction with the shared cache:</p>
<ul>
<li>
<p><code>jakarta.persistence.cache.retrieveMode</code></p>
</li>
<li>
<p><code>jakarta.persistence.cache.storeMode</code></p>
</li>
</ul>
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<h2 class="sect2"><font face="arial, helvetica, sans-serif" color="#330099">EclipseLink Extensions to Cache Query Hints</font></h2>
<p>The EclipseLink cache query hints allow for queries or the <code>find()</code> operation to interact with the cache is the following ways:</p>
<ul>
<li>
<p>Bypass the cache check and force accessing the database, but still resolve with the cache.</p>
</li>
<li>
<p>Refresh the cache from the database results.</p>
</li>
<li>
<p>Bypass the cache and persistence unit and return detached objects.</p>
</li>
<li>
<p>Bypass the persistence context and return read-only objects.</p>
</li>
<li>
<p>Allow queries that use Id fields, and other fields to obtain cache hits.</p>
</li>
<li>
<p>Query the cache first, and only access the database if the object is not found.</p>
</li>
<li>
<p>Only query the cache, and avoid accessing the database.</p>
</li>
<li>
<p>Conform a query with non-flushed changes in a persistence context.</p>
</li>
</ul>
<p>Queries that access the cache have the following restrictions:</p>
<ul>
<li>
<p>Sub-selects are not supported.</p>
</li>
<li>
<p>Certain database functions are not supported.</p>
</li>
<li>
<p>Queries must return a single set of objects.</p>
</li>
<li>
<p>Grouping is not supported.</p>
</li>
<li>
<p>Uninstantiated lazy relationships may not be able to be queried.</p>
</li>
</ul>
<p>All EclipseLink query hints are defined in the <code>QueryHints</code> class in the <code>org.eclipse.persistence.config</code> package. When you set a hint, you can set the value using the <code>public</code> <code>static</code> <code>final</code> field in the appropriate configuration class in <code>org.eclipse.persistence.config</code> package, including the following:</p>
<ul>
<li>
<p><code>HintValues</code></p>
</li>
<li>
<p><code>CacheUsage</code></p>
</li>
<li>
<p><code>PessimisticLock</code></p>
</li>
<li>
<p><code>QueryType</code></p>
</li>
</ul>
<p>You can specify EclipseLink query hints (JPA query extensions) either by using the <code>@QueryHint</code> annotation, by including the hints in the <code>orm.xml</code> or <code>eclipselink-orm.xml</code> files. or by using the <code>setHint()</code> method when executing a named or dynamic query (JPQL or Criteria).</p>
<p>Query settings and query hints that affect the generated SQL are not supported with SQL queries. Unsupported query hints include:</p>
<ul>
<li>
<p><code>batch</code></p>
</li>
<li>
<p><code>history.as-of</code></p>
</li>
<li>
<p><code>inheritance.outer-join</code></p>
</li>
<li>
<p><code>sql.hint</code></p>
</li>
<li>
<p><code>join-fetch</code>&mdash;<code>join-fetch</code> is supported, but requires that the SQL selects all of the joined columns.</p>
</li>
<li>
<p><code>fetch-group</code>&mdash;<code>fetch-group</code> is supported, but requires that the SQL selects all of the fetched columns.</p>
</li>
<li>
<p><code>pessimistic-lock</code>&mdash;<code>pessimistic-lock</code> is supported, but requires that the SQL locks the result rows.</p>
</li>
</ul>
<p>For descriptions of these extensions, see "EclipseLink Query Language" in <em>Jakarta Persistence API (JPA) Extensions Reference for EclipseLink</em>.</p>
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