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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
    </build-info>


<item>
            <title>[SERVER-85424] $group relies on stable sorting</title>
                <link>https://jira.mongodb.org/browse/SERVER-85424</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;We would like to use &lt;tt&gt;std::sort&lt;/tt&gt; (unstable sorting) instead of &lt;tt&gt;std::stable_sort&lt;/tt&gt; in sorter.cpp because of the performance gains. While experimenting with this change, I found that &lt;tt&gt;$group&lt;/tt&gt; with &lt;tt&gt;$first&lt;/tt&gt; relies on stable sorting. With unstable sorting, the behavior does not match the documentation at &lt;a href=&quot;https://www.mongodb.com/docs/manual/reference/operator/aggregation/first/#behaviors,&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://www.mongodb.com/docs/manual/reference/operator/aggregation/first/#behaviors,&lt;/a&gt; specifically&lt;/p&gt;

&lt;p&gt;&amp;gt;&#160;To define the document order for&#160;&lt;tt&gt;$first&lt;/tt&gt;&#160;in other pipeline stages, add a preceding&#160;&lt;a href=&quot;https://www.mongodb.com/docs/manual/reference/operator/aggregation/sort/#mongodb-pipeline-pipe.-sort&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;$sort&lt;/tt&gt;&lt;/a&gt;&#160;stage.&lt;/p&gt;

&lt;p&gt;The sorter change is in &lt;a href=&quot;https://github.com/10gen/mongo/pull/17859&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/10gen/mongo/pull/17859&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;cc &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=david.percy%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;david.percy@mongodb.com&quot;&gt;david.percy@mongodb.com&lt;/a&gt; This may be related to &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-85337&quot; class=&quot;external-link&quot; rel=&quot;nofollow&quot;&gt;SERVER-85337&lt;/a&gt;.&lt;/p&gt;</description>
                <environment></environment>
        <key id="2550732">SERVER-85424</key>
            <summary>$group relies on stable sorting</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="13202">Works as Designed</resolution>
                                        <assignee username="david.percy@mongodb.com">David Percy</assignee>
                                    <reporter username="brad.cater@mongodb.com">Brad Cater</reporter>
                        <labels>
                    </labels>
                <created>Thu, 18 Jan 2024 22:41:57 +0000</created>
                <updated>Fri, 19 Jan 2024 13:50:02 +0000</updated>
                            <resolved>Fri, 19 Jan 2024 13:50:02 +0000</resolved>
                                                                                        <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="6025497" author="JIRAUSER1276095" created="Fri, 19 Jan 2024 13:50:02 +0000"  >&lt;p&gt;Closing this because the suggested repro demonstrates the intended behavior, not a bug.&lt;/p&gt;</comment>
                            <comment id="6024430" author="david.percy" created="Fri, 19 Jan 2024 00:12:11 +0000"  >&lt;p&gt;Stable vs unstable sorting only affects how ties are handled, and in this example there are no ties (every document has a unique combination of {b, c} values).&#160; So I don&apos;t think this sorter change could have affected this example.&lt;/p&gt;

&lt;p&gt;$group+$first does rely on &quot;predictable ordering&quot; but not &quot;stable sorting&quot;.&#160; $first picks whichever input comes first, so its output is only predictable if the input order is predictable.&#160; $sort is unpredictable when there are ties, because different query plans / different indexes may break ties differently.&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;
&lt;hr /&gt;
&lt;p&gt;The behavior in the example looks correct to me. The {a: 2} group contains these docs:&lt;/p&gt;
&lt;p/&gt;
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&lt;p/&gt;
&lt;p&gt;so the only choices for &apos;c&apos; are 30 or 40.&#160; $first picks the {b: 10, c: 30} document because it sorts earlier by {b:1, c:1}.&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;To double check I ran the example on a branch with no sorter changes:&lt;/p&gt;
&lt;p/&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&amp;gt; db.coll.aggregate([ &#160; {$sort: {b: 1, c: 1,}}, &#160; {$group: {_id: &quot;$a&quot;, f: {$first: &quot;$c&quot;}}} ])&lt;/span&gt;&lt;/pre&gt;
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&lt;tt&gt;&#160; {a: 2, b: 10, c: 30&lt;/tt&gt;}&lt;br/&gt;
&lt;tt&gt;])&lt;/tt&gt;&lt;br/&gt;
{{{}}&lt;br/&gt;
&lt;tt&gt;&#160; &#160; &#160; &#160; &quot;acknowledged&quot; : true,&lt;/tt&gt;&lt;br/&gt;
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&lt;tt&gt;&#160; &#160; &#160; &#160; ]&lt;/tt&gt;&lt;br/&gt;
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&lt;tt&gt;&#160; {$group: {_id: &quot;$a&quot;, f: {$first: &quot;$c&quot;&lt;/tt&gt;}}}&lt;br/&gt;
&lt;tt&gt;])&lt;/tt&gt;&lt;br/&gt;
{{&lt;/p&gt;
{ &quot;_id&quot; : 1, &quot;f&quot; : 3 }
&lt;p&gt;}}&lt;br/&gt;
{{&lt;/p&gt;
{ &quot;_id&quot; : 2, &quot;f&quot; : 30 }
&lt;p&gt; &amp;lt;---- This should be {&quot;f&quot;: 20} because we sorted by &lt;span class=&quot;error&quot;&gt;&amp;#91;b, c&amp;#93;&lt;/span&gt;.}}&lt;/p&gt;</customfieldvalue>

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