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<item>
            <title>[SERVER-35904] Implement logic to detect if merging half of pipeline conserves but possibly renames the shard key</title>
                <link>https://jira.mongodb.org/browse/SERVER-35904</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;For pipelines like the following:&#160;&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;&#160; {$unwind: &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;$users&quot;&lt;/span&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;p/&gt;

&lt;p&gt;The ideal execution plan would be to perform the $unwind and a partial group on each shard, then scatter the partial groups by _id to merge in parallel on each shard. Supposing &quot;users&quot; was sharded with the shard key pattern {&lt;tt&gt;username: 1&lt;/tt&gt;}, we should distribute each partial group to the shard that would own that range of usernames.&lt;/p&gt;

&lt;p&gt;For example, suppose we had two shards: A and B. The &quot;teams&quot; collection is sharded and has some chunks on each shard. The &quot;users&quot; collection is sharded and has the chunk [MinKey, &quot;example&quot;) on shard A and &lt;span class=&quot;error&quot;&gt;&amp;#91;&amp;quot;example&amp;quot;, MaxKey&amp;#93;&lt;/span&gt; on shard B. Then we should execute this plan by having each shard perform a partial group on the teams collection, then send the partial groups with _id in the range [MinKey, &quot;example&quot;) to shard A, and the partial groups with _id in the range &lt;span class=&quot;error&quot;&gt;&amp;#91;&amp;quot;example&amp;quot;, MaxKey&amp;#93;&lt;/span&gt; to shard B. That way, in the absence of any chunk migrations, the final $out stage will perform a local write.&lt;/p&gt;

&lt;p&gt;In order to detect that partitioning by the shard key in this way is a good idea, we&apos;ll need to do some analysis of the merging half of the pipeline. It&apos;ll look something like this:&lt;/p&gt;
&lt;ol&gt;
	&lt;li&gt;Check if the last stage is a $out to a sharded collection&lt;/li&gt;
	&lt;li&gt;If so, consult the chunk manager to find the shard key and the routing table.&lt;/li&gt;
	&lt;li&gt;Work backwards through the merging half of the pipeline, tracking if any of the shard key fields are modified. If they are renamed, remember their new name.&lt;/li&gt;
	&lt;li&gt;If we get to the beginning of the merging pipeline without anyone modifying the fields, output the routing table and which fields should be used to partition the data.&lt;/li&gt;
&lt;/ol&gt;


&lt;p&gt;Once we do this, we can take advantage of this machinery and the work in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-35899&quot; title=&quot;Have mongos produce an aggregation plan that correctly uses an $exchange&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-35899&quot;&gt;&lt;del&gt;SERVER-35899&lt;/del&gt;&lt;/a&gt; to set up an $exchange across all the shards.&lt;/p&gt;</description>
                <environment></environment>
        <key id="565462">SERVER-35904</key>
            <summary>Implement logic to detect if merging half of pipeline conserves but possibly renames the shard key</summary>
                <type id="3" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14718&amp;avatarType=issuetype">Task</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="13201">Fixed</resolution>
                                        <assignee username="charlie.swanson@mongodb.com">Charlie Swanson</assignee>
                                    <reporter username="charlie.swanson@mongodb.com">Charlie Swanson</reporter>
                        <labels>
                    </labels>
                <created>Thu, 28 Jun 2018 22:08:58 +0000</created>
                <updated>Sun, 29 Oct 2023 22:30:14 +0000</updated>
                            <resolved>Fri, 24 Aug 2018 13:49:53 +0000</resolved>
                                                    <fixVersion>4.1.3</fixVersion>
                                    <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="1984775" author="charlie.swanson" created="Fri, 24 Aug 2018 13:48:03 +0000"  >
&lt;p&gt;Author:&lt;/p&gt;

{&apos;name&apos;: &apos;Charlie Swanson&apos;, &apos;email&apos;: &apos;charlie.swanson@mongodb.com&apos;, &apos;username&apos;: &apos;cswanson310&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-35905&quot; title=&quot;Plug pieces together to perform a distributed $exchange when applicable&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-35905&quot;&gt;&lt;del&gt;SERVER-35905&lt;/del&gt;&lt;/a&gt; Add logic to detect when $out is eligible for an exchange&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/7987c50c9702bf2280ca4f902499f58f98448241&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/7987c50c9702bf2280ca4f902499f58f98448241&lt;/a&gt;&lt;/p&gt;</comment>
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