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    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-50597] Explain against a pipeline containing $unionWith does not accurately count executionStats from sub-pipeline</title>
                <link>https://jira.mongodb.org/browse/SERVER-50597</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;When explaining a pipeline containing a $unionWith stage, we first &lt;a href=&quot;https://github.com/mongodb/mongo/blob/4406491b2b137984c2583db98068b7d18ea32171/src/mongo/db/query/explain.cpp#L811-L815&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;exhaust the pipeline to gather the execution stats&lt;/a&gt;. For union, this may or may not involve building and targetting its sub-pipeline depending on the subsequent stages (e.g. $limit may allow us to stream results directly from the base collection without using the pipeline). Next, we will serialize the $unionWith stage, at which point we &quot;re-run&quot; the pipeline and gather the explain output for it.&lt;/p&gt;

&lt;p&gt;There are separate issues with this depending on whether we&apos;re running in a sharded cluster:&lt;/p&gt;

&lt;p&gt;1. If &lt;b&gt;not&lt;/b&gt;&#160;sharded: the executionStats reported for the inner pipeline will not take into account subsequent stages such as $limit which could affect the nReturned stat.&lt;br/&gt;
 2. If sharded: The initial pass to exhaust the pipeline does not actually pull from the sub-pipeline since &lt;a href=&quot;https://github.com/mongodb/mongo/blob/4406491b2b137984c2583db98068b7d18ea32171/src/mongo/db/pipeline/sharded_agg_helpers.cpp#L872-L897&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;the helper we delegate to&lt;/a&gt; will see the explain bit and run a scatter-gather to get the results from each shard instead of establishing cursors to iterate. This means that the union stage essentially gets a no-op pipeline back, and the executionStats reported by the union stage itself will not include any potential documents from the sub-pipeline.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1454852">SERVER-50597</key>
            <summary>Explain against a pipeline containing $unionWith does not accurately count executionStats from sub-pipeline</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="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="nicholas.zolnierz@mongodb.com">Nicholas Zolnierz</reporter>
                        <labels>
                            <label>qopt-team</label>
                    </labels>
                <created>Thu, 27 Aug 2020 20:48:15 +0000</created>
                <updated>Wed, 2 Aug 2023 13:47:04 +0000</updated>
                                                                                                <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                    <issuelinks>
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            <issuekey id="1832379">SERVER-58888</issuekey>
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                                        <issuelink>
            <issuekey id="1437981">SERVER-50246</issuekey>
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            <issuelink>
            <issuekey id="2293850">SERVER-75108</issuekey>
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