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        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-20987] Incorrect or missing merge sort in mongos</title>
                <link>https://jira.mongodb.org/browse/SERVER-20987</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Got an issue from a customer that we were able to reproduce in 2.4, 2.6 and 3.0.&lt;/p&gt;

&lt;p&gt;4 clusters return each one document, however a request sorted does not seem to happen in the &lt;em&gt;mongos&lt;/em&gt; process.&lt;br/&gt;
Note that the field to sort is a sub field in an array, however each document only have one entry in the array.&lt;/p&gt;</description>
                <environment></environment>
        <key id="234869">SERVER-20987</key>
            <summary>Incorrect or missing merge sort in mongos</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="9">Done</resolution>
                                        <assignee username="randolph@mongodb.com">Randolph Tan</assignee>
                                    <reporter username="daniel.coupal@mongodb.com">Daniel Coupal</reporter>
                        <labels>
                    </labels>
                <created>Sat, 17 Oct 2015 00:32:34 +0000</created>
                <updated>Sun, 20 Mar 2016 13:05:43 +0000</updated>
                            <resolved>Sun, 20 Mar 2016 13:05:43 +0000</resolved>
                                                                    <component>Querying</component>
                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>8</watches>
                                                                                                                <comments>
                            <comment id="1064177" author="david.storch" created="Mon, 19 Oct 2015 14:22:50 +0000"  >&lt;p&gt;I haven&apos;t tracked down exactly why this is broken in existing stable versions, but I can confirm that this has been fixed as part of the mongos read path refactor done under SERVER-15176, so this issue will not affect 3.2. In the old mongos query code, mongos would extract the sort key from the result documents and use this for its merge sort. Something in this process appears to be broken.&lt;/p&gt;

&lt;p&gt;The new query path instead issues a {$meta: &quot;sortKey&quot;} projection to the shards. (Sort key meta-projection was added in the 3.1 development cycle explicitly for this purpose.) This instructs each shard to return the sort key it generated for each result document as part of its response. Mongos then uses this sort key metadata to perform its mergesort. It then strips out the metadata before returning the result set to the client. This work was done under &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-19355&quot; title=&quot;Support skip, limit, and projection in new find/getMore commands path on mongos&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-19355&quot;&gt;&lt;del&gt;SERVER-19355&lt;/del&gt;&lt;/a&gt;.&lt;/p&gt;</comment>
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