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    <language>en-us</language>    <build-info>
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        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-24518] MERGE_SORT_STAGE can be used more aggressively when OR_STAGE index sort orders match</title>
                <link>https://jira.mongodb.org/browse/SERVER-24518</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Currently, we only replace an OR_STAGE with a MERGE_SORT_STAGE when the sort order of the underlying indexes are &lt;b&gt;shared&lt;/b&gt; and match the &lt;b&gt;desired&lt;/b&gt; sort order of the query.&lt;/p&gt;

&lt;p&gt;The OR_STAGE uses unbounded memory to de-duplicate records.  The MERGE_SORT_STAGE currently seems to do the same, but could be optimized to use far less memory than the OR_STAGE since duplicate records are coming in from child stages in the same order.&lt;/p&gt;

&lt;p&gt;Given that MERGE_SORT_STAGE can be optimized to avoid unbounded memory, we should consider aggressively using it instead of OR_STAGEs, and re-SORTing afterwards (which happens in the OR_STAGE case as well).&lt;/p&gt;</description>
                <environment></environment>
        <key id="293006">SERVER-24518</key>
            <summary>MERGE_SORT_STAGE can be used more aggressively when OR_STAGE index sort orders match</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="10038" iconUrl="https://jira.mongodb.org/images/icons/subtask.gif" description="">Backlog</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-query-optimization">Backlog - Query Optimization</assignee>
                                    <reporter username="david.hatch">David Hatch</reporter>
                        <labels>
                            <label>neweng</label>
                            <label>qexec-team</label>
                    </labels>
                <created>Fri, 10 Jun 2016 15:38:13 +0000</created>
                <updated>Tue, 31 Oct 2023 17:01:20 +0000</updated>
                                                                            <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>12</watches>
                                                                                                                <comments>
                            <comment id="1291129" author="david.storch" created="Fri, 10 Jun 2016 21:48:42 +0000"  >&lt;p&gt;Right, good point, I think we would have to clear MergeSortStage::_seen every time the value of the sort key of the documents being merged changes.&lt;/p&gt;</comment>
                            <comment id="1290648" author="david.hatch" created="Fri, 10 Jun 2016 16:04:29 +0000"  >&lt;p&gt;Yeah, and I think we&apos;d be able to reduce memory usage in the MERGE_SORT stage, right?&lt;/p&gt;</comment>
                            <comment id="1290634" author="david.storch" created="Fri, 10 Jun 2016 15:54:45 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=david.hatch&quot; class=&quot;user-hover&quot; rel=&quot;david.hatch&quot;&gt;david.hatch&lt;/a&gt;, are you saying that we should use a MERGE_SORT stage when possible, even if the sort obtained by the MERGE_SORT does not satisfy the user requested sort (and hence a subsequent SORT stage would be required) and even if the user did not request a sort? If I understand correctly, it&apos;s a very interesting idea &lt;img class=&quot;emoticon&quot; src=&quot;https://jira.mongodb.org/images/icons/emoticons/smile.png&quot; height=&quot;16&quot; width=&quot;16&quot; align=&quot;absmiddle&quot; alt=&quot;&quot; border=&quot;0&quot;/&gt;&lt;/p&gt;</comment>
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            <issuekey id="344965">SERVER-27658</issuekey>
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            <issuekey id="767920">SERVER-41142</issuekey>
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