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POHow to avoid unnecessary selects and joins in HQL and Criteria
 

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  1. POHow to avoid unnecessary selects and joins in HQL and Criteria
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    <p>I have been trying different combinations of <strong>HQL</strong> and <strong>Criteria</strong> and I haven't been able to avoid some unnecessary <strong>joins</strong> (in both) and some unnecessary <strong>selects</strong> (in Criteria).</p> <p>In our scenario, we have a <strong>@ManyToMany</strong> relationship between <strong>Segment</strong> and <strong>Application</strong> entities (navigation is from Segment to Applications).</p> <p>First I tried this <strong>Criteria</strong>:</p> <pre><code>Application app = ... List&lt;Segment&gt; segments = session.createCriteria(Segment.class) .createCriteria(Segment.APPLICATIONS) .add(Restrictions.idEq(app.getId())) .list(); </code></pre> <p>Wich produces this SQL:</p> <pre><code>select this_.id as id1_1_, this_.description as descript2_1_1_, this_.name as name1_1_, applicatio3_.segment_id as segment1_1_, applicatio1_.id as app2_, &lt;==== unnecessary APPLICATIONS columns applicatio1_.id as id7_0_, applicatio1_.name as name7_0_, applicatio1_.accountId as accountId7_0_, applicatio1_.applicationFlags as applicat5_7_0_, applicatio1_.description_ as descript6_7_0_, from SEGMENTS this_ inner join SEGMENTS_APPLICATIONS applicatio3_ on this_.id=applicatio3_.segment_id inner join &lt;==== unnecessary join APPLICATIONS applicatio1_ on applicatio3_.app_id=applicatio1_.id where applicatio1_.id = ? </code></pre> <p>As you can see, <em>Criteria selects columns from APPLICATIONS</em>, which I don't want to be selected. I haven't found a way to do it (is it possible?). Also, it <em>joins with APPLICATIONS</em>, which I think is not necessary because the application id is already in the join table SEGMENTS_APPLICATIONS (the same happens with HQL).</p> <p>(As an additional doubt, I'd like to know a Restriction that uses the app directly, and not app.getId(). As you will see, I could do that in the HQL version of the query)</p> <p>Since I couldn't limit the select part (I don't need Application properties) I tried this <strong>HQL</strong> with the "select" clause:</p> <pre><code>Application app = ... List&lt;Segment&gt; segments = session.createQuery( "select s from Segment s join s.applications as app where app = :app") .setParameter("app", app) .list(); </code></pre> <p>wich produces:</p> <pre><code>select segment0_.id as id1_, segment0_.description as descript2_1_, segment0_.name as name1_, from SEGMENTS segment0_ inner join SEGMENTS_APPLICATIONS applicatio1_ on segment0_.id=applicatio1_.segment_id inner join &lt;==== unnecessary join APPLICATIONS applicatio2_ on applicatio1_.app_id=applicatio2_.id where applicatio2_.id=? </code></pre> <p>You can see the <em>HQL doesn't select properties from Application</em> (thanks to the "select s" part), but <em>still joins the APPLICATIONS</em> table, which I think is unnecessary. How can we avoid that?</p> <p>(As a side note, notice that in HQL I could use app directly, and not app.getId() like in the Criteria)</p> <p>Can you please help me find a way to avoid "selects" in Criteria and unnecessary "joins" in both Criteria and HQL?</p> <p>(This example is with @ManyToMany but I think it also happens with @OneToMany and also with @ManyToOne and @OneToOne, even with fetch = LAZY).</p> <p>Thank you very much, Ferran</p>
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