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    <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>[DOCS-12781] [Server] Document that explain neither reads from nor writes to the plan cache</title>
                <link>https://jira.mongodb.org/browse/DOCS-12781</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;h2&gt;&lt;a name=&quot;Description&quot;&gt;&lt;/a&gt;Description&lt;/h2&gt;

&lt;p&gt;When users run an explain of any command (find, aggregate, update, etc.), the plan cache is not used. Instead, the set of candidate plans are generated and a winner chosen without ever consulting the plan cache as described here: &lt;a href=&quot;https://docs.mongodb.com/manual/core/query-plans/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://docs.mongodb.com/manual/core/query-plans/&lt;/a&gt;. Furthermore, no plan cache entry is created for the winning plan. We should document this behavior.&lt;/p&gt;

&lt;p&gt;This is a common source of user questions and confusion, because when a cache entry is present it can cause users to observe a discrepancy in the plans selected by explain and the plans selected without explain. &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-16895&quot; title=&quot;Users should be able to request that explain not bypass the plan cache&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-16895&quot;&gt;SERVER-16895&lt;/a&gt; is an open request to allow users to run explain in a mode which read a plan out of the plan cache, but this improvement is not currently scheduled for development.&lt;/p&gt;
&lt;h2&gt;&lt;a name=&quot;Scopeofchanges&quot;&gt;&lt;/a&gt;Scope of changes&lt;/h2&gt;
&lt;ul&gt;
	&lt;li&gt;Clarify stated behavior &lt;tt&gt;in:&lt;/tt&gt;
	&lt;ul&gt;
		&lt;li&gt;&lt;tt&gt;&lt;a href=&quot;https://docs.mongodb.com/master/reference/command/explain/index.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;explain dbcommand&lt;/a&gt;&lt;/tt&gt;&lt;/li&gt;
		&lt;li&gt;&lt;tt&gt;&lt;a href=&quot;https://docs.mongodb.com/master/reference/method/db.collection.explain/#db.collection.explain&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;db.collection.explain()&lt;/a&gt;&lt;/tt&gt;&lt;/li&gt;
		&lt;li&gt;&lt;tt&gt;&lt;a href=&quot;https://docs.mongodb.com/master/reference/method/cursor.explain/#cursor.explain&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;cursor.explain()&lt;/a&gt;&lt;/tt&gt;&lt;/li&gt;
		&lt;li&gt;&lt;tt&gt;&lt;a href=&quot;https://docs.mongodb.com/master/reference/operator/meta/explain/index.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;$explain (deprecated)&lt;/a&gt;&lt;/tt&gt;&lt;/li&gt;
	&lt;/ul&gt;
	&lt;/li&gt;
&lt;/ul&gt;


&lt;h2&gt;&lt;a name=&quot;%C2%A0&quot;&gt;&lt;/a&gt;&#160;&lt;/h2&gt;</description>
                <environment></environment>
        <key id="791794">DOCS-12781</key>
            <summary>[Server] Document that explain neither reads from nor writes to the plan cache</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="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="-1">Unassigned</assignee>
                                    <reporter username="david.storch@mongodb.com">David Storch</reporter>
                        <labels>
                            <label>backlog</label>
                            <label>feature</label>
                            <label>query</label>
                            <label>reopened</label>
                            <label>server-docs-bug-bash</label>
                    </labels>
                <created>Thu, 6 Jun 2019 17:56:12 +0000</created>
                <updated>Mon, 22 Jan 2024 15:53:57 +0000</updated>
                                                                            <component>manual</component>
                        <due></due>
                            <votes>0</votes>
                                    <watches>7</watches>
                                                                                                                <comments>
                            <comment id="4947911" author="JIRAUSER1264463" created="Wed, 2 Nov 2022 13:45:59 +0000"  >&lt;p&gt;Thanks &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=david.storch%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;david.storch@mongodb.com&quot;&gt;david.storch@mongodb.com&lt;/a&gt;! Appreciate the confirmation.&#160;&lt;/p&gt;</comment>
                            <comment id="4947720" author="david.storch" created="Wed, 2 Nov 2022 13:20:29 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=sarah.olson%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;sarah.olson@mongodb.com&quot;&gt;sarah.olson@mongodb.com&lt;/a&gt; &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=ashley.brown%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;ashley.brown@mongodb.com&quot;&gt;ashley.brown@mongodb.com&lt;/a&gt; I&apos;m re-opening this issue. It still seems relevant. The fact that explain does not use the plan cache can sometimes be relevant when debugging query performance, so it seems worth extending the docs to mention this at some point.&lt;/p&gt;</comment>
                            <comment id="4940461" author="edu.bot" created="Mon, 31 Oct 2022 16:05:49 +0000"  >&lt;p&gt;Hello! This ticket has been closed due to inactivity. If you believe this ticket is still important, please reopen it and leave a comment to explain why. Thank you!&lt;/p&gt;</comment>
                    </comments>
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            <issuekey id="745541">SERVER-40844</issuekey>
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