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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-34453] aggregation $count underperforms count()</title>
                <link>https://jira.mongodb.org/browse/SERVER-34453</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;i have a collection with 10m rows.&lt;/p&gt;

&lt;p&gt;this returns in like 10ms:&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.getCollection(&apos;people&apos;).count()&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
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&lt;/table&gt;
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&lt;p/&gt;
&lt;p&gt;while this returns like 10s:&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.getCollection(&apos;people&apos;).aggregate([{$count: &apos;count&apos;}])&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;doesn&apos;t seem to be related to &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-7568&quot; class=&quot;external-link&quot; rel=&quot;nofollow&quot;&gt;https://jira.mongodb.org/browse/SERVER-7568&lt;/a&gt;, but similar in marked performance diff doing similar ops in aggregation and non-aggregation styles.&lt;/p&gt;</description>
                <environment></environment>
        <key id="526352">SERVER-34453</key>
            <summary>aggregation $count underperforms count()</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="4" iconUrl="https://jira.mongodb.org/images/icons/priorities/minor.svg">Minor - P4</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="13202">Works as Designed</resolution>
                                        <assignee username="william.byrne@mongodb.com">William Byrne III</assignee>
                                    <reporter username="tony-kerz">tony kerz</reporter>
                        <labels>
                    </labels>
                <created>Fri, 13 Apr 2018 02:39:55 +0000</created>
                <updated>Fri, 27 Oct 2023 13:53:51 +0000</updated>
                            <resolved>Fri, 25 May 2018 22:24:00 +0000</resolved>
                                    <version>3.7.3</version>
                                                    <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>9</watches>
                                                                                                                <comments>
                            <comment id="1878817" author="william.byrne" created="Tue, 1 May 2018 07:56:53 +0000"  >&lt;p&gt;Hi Tony,&lt;/p&gt;

&lt;p&gt;The reason for these performance differences is that a &lt;a href=&quot;https://docs.mongodb.com/manual/reference/method/db.collection.count/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;count()&lt;/a&gt;&#160;without a predicate doesn&apos;t fetch the matching documents and actually count them, it uses the metadata for the collection to return the number count of documents that &lt;b&gt;would&lt;/b&gt; match an equivalent find() query. In contrast, the &lt;tt&gt;$count&lt;/tt&gt; aggregation stage iterates through the documents passed to it from the prior stage (or from the collection/index scan if it is the first stage) and counts them.&lt;/p&gt;

&lt;p&gt;The equivalent non-aggregation command for &lt;tt&gt;$count&lt;/tt&gt; is actually &lt;a href=&quot;https://docs.mongodb.com/manual/reference/method/cursor.itcount/index.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;itcount&lt;/a&gt;, as it also iterates through the matching documents. You will find &lt;tt&gt;$count&lt;/tt&gt; and &lt;tt&gt;itcount()&lt;/tt&gt; have similar performance.&lt;/p&gt;

&lt;p&gt;Note also that while just reading the collection metadata as &lt;tt&gt;count()&lt;/tt&gt; does is faster, it can &lt;a href=&quot;https://docs.mongodb.com/manual/reference/method/db.collection.count/#sharded-clusters&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;return inaccurate counts for sharded clusters&lt;/a&gt; if there are orphaned documents or ongoing chunk migrations. Both &lt;tt&gt;$count&lt;/tt&gt; and &lt;tt&gt;itcount()&lt;/tt&gt; queries pull the documents to be counted through the SHARDING_FILTER, which eliminates duplicated documents from failed and ongoing migrations, so their results are accurate.  &lt;/p&gt;

&lt;p&gt;Regards,&lt;/p&gt;

&lt;p&gt;William Byrne III&lt;/p&gt;</comment>
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