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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>
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<item>
            <title>[SERVER-51795] Explain command on sharded clusters does not support readConcern</title>
                <link>https://jira.mongodb.org/browse/SERVER-51795</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;On standalones, the explain command does not support readConcern, as it uses the default implementation of &lt;tt&gt;supportsReadConcern()&lt;/tt&gt; (see &lt;a href=&quot;https://github.com/mongodb/mongo/blob/306f04029f01d3bfb7bc565fb2139862daa57397/src/mongo/db/commands.h#L592-L599&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;here&lt;/a&gt;). For example&lt;/p&gt;

&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&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;  margin-top: 10px;   margin-bottom: 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;db.c.runCommand({explain: {find: &quot;c&quot;, query: {a: 1}}, readConcern: {level: &quot;available&quot;}}) // Error&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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			&lt;/tbody&gt;
&lt;/table&gt;
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&lt;p/&gt;

&lt;p&gt;It is, however, legal to provide a readConcern in the inner command. For example:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&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;  margin-top: 10px;   margin-bottom: 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;db.c.runCommand({explain: {count: &quot;c&quot;, query: {a: 1}, readConcern: {level: &quot;available&quot;}}})&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;When running explain on a sharded cluster, mongos will &quot;hoist&quot; generic options from the inner command into the top-level &quot;explain&quot; command which it sends to the shards. See &lt;a href=&quot;https://github.com/mongodb/mongo/blob/69c03caa427e1d31b81e7bc98a56d9eb7e6ab06b/src/mongo/s/commands/cluster_explain.cpp#L125-L129&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;here&lt;/a&gt;). The result of this is that attempting to explain a query with a readConcern other than &quot;local&quot; on mongos throws an error.&lt;/p&gt;

&lt;p&gt;It is possible to get different query plans using different readConcern levels, and it would be extremely useful if explain() could reflect this. For example, queries run with readConcern &quot;available&quot; do not perform orphan filtering, and do not need a &lt;tt&gt;SHARD_FILTER&lt;/tt&gt; stage. In many cases, this can lead to a completely different plan being used than if the query had been run with a stronger readConcern.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1521296">SERVER-51795</key>
            <summary>Explain command on sharded clusters does not support readConcern</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="13201">Fixed</resolution>
                                        <assignee username="henri.nikku@mongodb.com">Henri Nikku</assignee>
                                    <reporter username="ian.boros@mongodb.com">Ian Boros</reporter>
                        <labels>
                            <label>quick-tech-debt</label>
                    </labels>
                <created>Wed, 21 Oct 2020 20:40:04 +0000</created>
                <updated>Fri, 17 Nov 2023 11:52:40 +0000</updated>
                            <resolved>Fri, 17 Nov 2023 11:52:39 +0000</resolved>
                                                    <fixVersion>7.3.0-rc0</fixVersion>
                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>16</watches>
                                                                                                                <comments>
                            <comment id="5887772" author="xgen-internal-githook" created="Fri, 17 Nov 2023 11:16:44 +0000"  >&lt;p&gt;Author: &lt;/p&gt;
{&apos;name&apos;: &apos;henrinikku&apos;, &apos;email&apos;: &apos;henri.nikku@mongodb.com&apos;, &apos;username&apos;: &apos;henrinikku&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-51795&quot; title=&quot;Explain command on sharded clusters does not support readConcern&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-51795&quot;&gt;&lt;del&gt;SERVER-51795&lt;/del&gt;&lt;/a&gt; Allow explain for all read concern levels&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/feaffd83c4fe443a1568cc2ec543e9849c0c56ff&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/feaffd83c4fe443a1568cc2ec543e9849c0c56ff&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="4350660" author="JIRAUSER1257467" created="Fri, 11 Feb 2022 16:19:56 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=jacob.evans&quot; class=&quot;user-hover&quot; rel=&quot;jacob.evans&quot;&gt;jacob.evans&lt;/a&gt;, just wondering if do you remember what was the decision to close this PR?&lt;/p&gt;</comment>
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