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<td class="page-title-pattern-first-line " style="padding: 0; border-collapse: collapse; font-family: Arial, sans-serif; font-size: 14px; padding-top: 10px"> <a href="https://hibernate.atlassian.net/browse/HHH" style="color: #3b73af; text-decoration: none">Hibernate ORM</a> / <a href="https://hibernate.atlassian.net/browse/HHH-10965" style="color: #3b73af; text-decoration: none"><img src="cid:jira-generated-image-avatar-45367831-7f34-4fba-9af9-4259321b0ef9" height="16" width="16" border="0" align="absmiddle" alt="New Feature" style="vertical-align: text-bottom"></a> <a href="https://hibernate.atlassian.net/browse/HHH-10965" style="color: #3b73af; text-decoration: none">HHH-10965</a> </td>
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<td style="vertical-align: top;; padding: 0; border-collapse: collapse; padding-right: 5px; font-size: 20px; line-height: 30px; mso-line-height-rule: exactly" class="page-title-pattern-header-container"> <span class="page-title-pattern-header" style="font-family: Arial, sans-serif; padding: 0; font-size: 20px; line-height: 30px; mso-text-raise: 2px; mso-line-height-rule: exactly; vertical-align: middle"> <a href="https://hibernate.atlassian.net/browse/HHH-10965" style="color: #3b73af; text-decoration: none">Add new Query Hint to avoid passing DISTINCT from entity queries to SQL statements</a> </span> </td>
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<th style="color: #707070; font: normal 14px/20px Arial, sans-serif; text-align: left; vertical-align: top; padding: 2px 0">Issue Type:</th>
<td class="has-icon" style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> <img src="cid:jira-generated-image-avatar-45367831-7f34-4fba-9af9-4259321b0ef9" height="16" width="16" border="0" align="absmiddle" alt="New Feature" style="vertical-align: text-bottom"> New Feature </td>
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<th style="color: #707070; font: normal 14px/20px Arial, sans-serif; text-align: left; vertical-align: top; padding: 2px 0">Affects Versions:</th>
<td style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> 5.2.1 </td>
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<td style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> <a class="user-hover" rel="mih_vlad" id="email_mih_vlad" href="https://hibernate.atlassian.net/secure/ViewProfile.jspa?name=mih_vlad" style="color:#6c797f;; color: #3b73af; text-decoration: none">Vlad Mihalcea</a> </td>
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<th style="color: #707070; font: normal 14px/20px Arial, sans-serif; text-align: left; vertical-align: top; padding: 2px 0">Created:</th>
<td style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> 18/Jul/2016 03:15 AM </td>
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<td class="has-icon" style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> <img src="cid:jira-generated-image-static-major-b417f6a7-d0f5-4296-9924-74642aa5a0f0" height="16" width="16" border="0" align="absmiddle" alt="Major" style="vertical-align: text-bottom"> Major </td>
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<th style="color: #707070; font: normal 14px/20px Arial, sans-serif; text-align: left; vertical-align: top; padding: 2px 0">Reporter:</th>
<td style="padding: 0; border-collapse: collapse; font: normal 14px/20px Arial, sans-serif; padding: 2px 0 2px 5px; vertical-align: top"> <a class="user-hover" rel="mih_vlad" id="email_mih_vlad" href="https://hibernate.atlassian.net/secure/ViewProfile.jspa?name=mih_vlad" style="color:#6c797f;; color: #3b73af; text-decoration: none">Vlad Mihalcea</a> </td>
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<td class="text-paragraph-pattern-container mobile-resize-text " style="padding: 0; border-collapse: collapse; padding: 0 0 10px"> <p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0; margin-top: 0">This issue has been documented on the <a href="http://lists.jboss.org/pipermail/hibernate-dev/2016-July/015086.html" class="external-link" rel="nofollow" style="color: #3b73af; text-decoration: none">mailing list</a>. </p> <p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0">Basically, the reason for this enhancement can be summarized as follows:</p>
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<p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0; margin-top: 0">In JPQL/HQL, the DISTINCT keyword is meant to avoid returning the same<br> parent entities when JOIN FETCHING parents with child associations.<br> JPA backs it up too:</p>
<p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0">"The DISTINCT keyword is used to specify that duplicate values must be<br> eliminated from the query result."</p>
<p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0">However, I see that the DISTINCT is applied on the SQL statement as well.</p>
<p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0">Why is it so? Is there any reason why we would want such a behaviour?<br> The problem is that an unnecessary DISTINCT might affect the execution plan:</p>
<p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0"><a href="http://use-the-index-luke.com/sql/join/nested-loops-join-n1-problem?langtype=java#orm-join" class="external-link" rel="nofollow" style="color: #3b73af; text-decoration: none">http://use-the-index-luke.com/sql/join/nested-loops-join-n1-problem?langtype=java#orm-join</a></p>
<p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0">"The distinct keyword in the SQL query is alarming because most databases<br> will actually filter duplicate records. Only a few databases recognize that<br> the primary keys guarantees uniqueness in that case anyway."</p>
<p style="margin-top:0;margin-bottom:10px;; margin: 10px 0 0">Should we provide a Query hint like HINT_DISTINCT_SQL which when explicitly<br> set, the JPQL DISTINCT keyword generates a DISTINCT keyword on the SQL<br> statement-level as well.<br> Without the hint set explicitly, we should not pass the DISTINCT to the SQL<br> statement.</p>
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