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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>[DOCS-14453] Investigate changes in SERVER-53762: Report aggregate execution stats in explain for the inner side of $lookup</title>
                <link>https://jira.mongodb.org/browse/DOCS-14453</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;div class=&quot;panel&quot; style=&quot;background-color: #c2d2c2;border-width: 1px;&quot;&gt;&lt;div class=&quot;panelHeader&quot; style=&quot;border-bottom-width: 1px;background-color: #239eb0;&quot;&gt;&lt;b&gt;Downstream Change Summary&lt;/b&gt;&lt;/div&gt;&lt;div class=&quot;panelContent&quot; style=&quot;background-color: #c2d2c2;&quot;&gt;
&lt;p&gt;In this ticket, we&apos;ve modified the explain output for aggregations using $lookup. If the explain verbosity is &lt;tt&gt;allPlansExecution&lt;/tt&gt; or above, we will add total aggregate execution stats to the output. (The specific stats are &lt;tt&gt;totalDocsExamined&lt;/tt&gt;, &lt;tt&gt;totalKeysExamined&lt;/tt&gt;, &lt;tt&gt;collectionScans&lt;/tt&gt; (the number as an integer), and &lt;tt&gt;indexesUsed&lt;/tt&gt; (an array of the index names).&lt;/p&gt;
&lt;/div&gt;&lt;/div&gt;
&lt;h2&gt;&lt;a name=&quot;DescriptionofLinkedTicket&quot;&gt;&lt;/a&gt;Description of Linked Ticket&lt;/h2&gt;

&lt;p&gt;As part of &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-47640&quot; title=&quot;$lookup should increment the serverStatus metrics.queryExecutor counters&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-47640&quot;&gt;&lt;del&gt;SERVER-47640&lt;/del&gt;&lt;/a&gt;, we enhanced the implementation of &lt;tt&gt;$lookup&lt;/tt&gt; to keep track of the &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e0c8a0bfdfc155443dcdae5bdb82575f4bfdb669/src/mongo/db/query/plan_summary_stats.h&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;PlanSummaryStats&lt;/tt&gt;&lt;/a&gt; associated with the subpipeline. These stats include metrics such as the number of keys examined and the number of documents examined by the subpipeline. Note that the subpipeline may be executed many times, since it is typically executed once per document from the outer (or &quot;local&quot;) side. The &lt;tt&gt;PlanSummaryStats&lt;/tt&gt; are tracked in aggregate across all such executions of the subpipeline. So if, for instance, the subpipeline scans a 10 document collection every time it runs, and it runs 5 times, then the result will be a &quot;docsExamined&quot; quantity of 50.&lt;/p&gt;

&lt;p&gt;Now that we are tracking these values, it would be valuable for debugging/diagnostics to expose them in &quot;executionStats&quot; or &quot;allPlansExecution&quot; level explain output. We will need to choose which metrics are exposed, but I think it would at the very least include docs examined and keys examined. This would be especially helpful in understanding the performance properties of the subpipeline, since explain currently does not display the execution plan used on the inner side (see &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-22622&quot; title=&quot;Improve $lookup explain to indicate query plan on the &amp;quot;from&amp;quot; collection&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-22622&quot;&gt;SERVER-22622&lt;/a&gt;). Any information in explain as to whether the $lookup subpipeline is cheap or expensive and why would be immensely helpful for query performance debugging scenarios.&lt;/p&gt;
&lt;h2&gt;&lt;a name=&quot;Scopeofchanges&quot;&gt;&lt;/a&gt;Scope of changes&lt;/h2&gt;

&lt;p&gt;Consider if we need to update the $lookup docs as part of this change.&lt;/p&gt;
&lt;h2&gt;&lt;a name=&quot;ImpacttoOtherDocs&quot;&gt;&lt;/a&gt;Impact to Other Docs&lt;/h2&gt;
&lt;h2&gt;&lt;a name=&quot;MVP%28WorkandDate%29&quot;&gt;&lt;/a&gt;MVP (Work and Date)&lt;/h2&gt;
&lt;h2&gt;&lt;a name=&quot;Resources%28ScopeorDesignDocs%2CInvision%2Cetc.%29&quot;&gt;&lt;/a&gt;Resources (Scope or Design Docs, Invision, etc.)&lt;/h2&gt;</description>
                <environment></environment>
        <key id="1715017">DOCS-14453</key>
            <summary>Investigate changes in SERVER-53762: Report aggregate execution stats in explain for the inner side of $lookup</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="9">Done</resolution>
                                        <assignee username="jason.price@mongodb.com">Jason Price</assignee>
                                    <reporter username="backlog-server-pm">Backlog - Core Eng Program Management Team</reporter>
                        <labels>
                    </labels>
                <created>Thu, 13 May 2021 20:47:04 +0000</created>
                <updated>Mon, 13 Nov 2023 17:54:35 +0000</updated>
                            <resolved>Mon, 23 Aug 2021 16:45:17 +0000</resolved>
                                                    <fixVersion>5.0.0-rc0</fixVersion>
                    <fixVersion>Server_Docs_20231030</fixVersion>
                    <fixVersion>Server_Docs_20231106</fixVersion>
                    <fixVersion>Server_Docs_20231105</fixVersion>
                    <fixVersion>Server_Docs_20231113</fixVersion>
                                    <component>manual</component>
                    <component>Server</component>
                        <due></due>
                            <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="4017389" author="xgen-internal-githook" created="Mon, 23 Aug 2021 16:42:08 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;jason-price-mongodb&apos;, &apos;email&apos;: &apos;jshfjghsdfgjsdjh@aolsdjfhkjsdhfkjsdf.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14453&quot; title=&quot;Investigate changes in SERVER-53762: Report aggregate execution stats in explain for the inner side of $lookup&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14453&quot;&gt;&lt;del&gt;DOCS-14453&lt;/del&gt;&lt;/a&gt; execution stats inner lookup&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14453&quot; title=&quot;Investigate changes in SERVER-53762: Report aggregate execution stats in explain for the inner side of $lookup&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14453&quot;&gt;&lt;del&gt;DOCS-14453&lt;/del&gt;&lt;/a&gt; execution stats inner lookup&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/872ec941df29fd4d0d2684b6fbbed8cf25d0bec5&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/872ec941df29fd4d0d2684b6fbbed8cf25d0bec5&lt;/a&gt;&lt;/p&gt;</comment>
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