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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-8996] $avg aggregation operator should return null instead of 0</title>
                <link>https://jira.mongodb.org/browse/DOCS-8996</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;p&gt;Currently, if you run $avg against a (nonexistent) field, the return will be 0, instead of null. If you do $min or $max, you will get a null. The reason $avg and $sum should behave this way is so that when you do two pipeline $group operators in a row, currently $avg does not behave the same in both pipelines because the first $group stage will return a 0 instead of null for records that don&apos;t have the $group _id.&lt;/p&gt;

&lt;p&gt;For example, imagine the records&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;   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;{&apos;a&apos; : 1, &apos;b&apos; : 1, &apos;value&apos; : 1}&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&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;   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;{&apos;a&apos; : 1, &apos;b&apos; : 1, &apos;value&apos; : 9}&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&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-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;{&apos;a&apos; : 1, &apos;b&apos; : 2}&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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&lt;/table&gt;
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&lt;p/&gt;
&lt;p&gt;then if I run&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.test.aggregate({$group : {&apos;_id&apos; : {&apos;a&apos; : &apos;$a&apos;, &apos;b&apos; : &apos;$b&apos;}, &apos;average&apos; : {&apos;$avg&apos; : &apos;$value&apos;}}}, {$group : {&apos;_id&apos; : {&apos;a&apos; : &apos;$_id.a&apos;}, &apos;average&apos; : {&apos;$avg&apos; : &apos;$average&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;I get 2.5 instead of 5, of course because the first $avg operation returns 0. If, however, this operation returned NULL, then it would work fine. Also, if this type of behavior is DESIRED, a simple ifnull() could always be used. However, I am of the opinion that $avg of a bunch of nulls is NULL, not 0.&lt;/p&gt;

&lt;p&gt;I suppose the $sum operation returning 0 instead of null is of less consequence, but it should probably behave the same way.&lt;/p&gt;

&lt;p&gt;Thoughts?&lt;/p&gt;</description>
                <environment></environment>
        <key id="319635">DOCS-8996</key>
            <summary>$avg aggregation operator should return null instead of 0</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="allison.moore@mongodb.com">Allison Reinheimer Moore</assignee>
                                    <reporter username="emily.hall">Emily Hall</reporter>
                        <labels>
                    </labels>
                <created>Wed, 28 Sep 2016 18:34:10 +0000</created>
                <updated>Mon, 30 Oct 2023 21:24:58 +0000</updated>
                            <resolved>Wed, 2 Aug 2017 15:48:25 +0000</resolved>
                                                    <fixVersion>Server_Docs_20231030</fixVersion>
                                    <component>Server</component>
                        <due></due>
                            <votes>0</votes>
                                    <watches>3</watches>
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