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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-31082] when $count is at the end of multiple stages that don&apos;t change the number of documents in pipeline, those stages can be eliminated</title>
                <link>https://jira.mongodb.org/browse/SERVER-31082</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Something that looks like this:&lt;/p&gt;

&lt;p/&gt;
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				&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;{$lookup:{}},&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{$addFields:{}},&lt;/span&gt;&lt;/pre&gt;
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				&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;{$lookup:{}},&lt;/span&gt;&lt;/pre&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{$count:&quot;c&quot;}&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;

&lt;p&gt;is basically a $match followed by a $count.  It would be nice if aggregation had a way of figuring that out.&lt;/p&gt;</description>
                <environment></environment>
        <key id="427201">SERVER-31082</key>
            <summary>when $count is at the end of multiple stages that don&apos;t change the number of documents in pipeline, those stages can be eliminated</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</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="backlog-query-optimization">Backlog - Query Optimization</assignee>
                                    <reporter username="asya.kamsky@mongodb.com">Asya Kamsky</reporter>
                        <labels>
                    </labels>
                <created>Wed, 13 Sep 2017 19:40:20 +0000</created>
                <updated>Thu, 19 Jan 2023 19:50:54 +0000</updated>
                                                                            <component>Aggregation Framework</component>
                                        <votes>1</votes>
                                    <watches>12</watches>
                                                                                                                <comments>
                            <comment id="4186902" author="david.percy" created="Fri, 12 Nov 2021 22:51:56 +0000"  >&lt;p&gt;Another stage that doesn&apos;t change the count is $sort.  We could recognize whether or not a $group uses order-sensitive accumulators ($first, $push).  Since we consider $sum / $add to be commutative and associative, we could eliminate the $sort in a pipeline like &lt;span class=&quot;error&quot;&gt;&amp;#91;{$sort ...} {$group: {_id: null, n: {$sum:1}}}&amp;#93;&lt;/span&gt;.&lt;/p&gt;</comment>
                            <comment id="4186857" author="JIRAUSER1253388" created="Fri, 12 Nov 2021 22:30:45 +0000"  >&lt;p&gt;I think this could be done by changing $count to be a concrete stage instead of an alias for $group $project. When&#160;&lt;br/&gt;
 stages like $addFields and $project run their doOptimize function, they will see a $count ahead and remove themselves from the pipeline. Then inside the $count doOptimize, we remove the $count from the pipeline and add $group $project. So $count will always result in a $group $project, but is a concrete type (although it will never show up in a pipeline once optimization is done).&lt;br/&gt;
 &#160;&lt;br/&gt;
 I think this would give the same effect as what we have now, except that stages can optimize if they see a $count.&lt;/p&gt;

&lt;p&gt;Also it looks like $graphLookup is another stage that does not change the document count.&lt;/p&gt;</comment>
                            <comment id="1672249" author="asya" created="Wed, 13 Sep 2017 19:41:36 +0000"  >&lt;p&gt;I&apos;m guessing that would be part of dependency analysis because this optimization would require figuring out that $count (or any $group which doesn&apos;t use any incoming fields) makes all such stages no-ops.&lt;/p&gt;</comment>
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                                                                <inwardlinks description="is related to">
                                        <issuelink>
            <issuekey id="2236909">SERVER-73027</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="132949">SERVER-13703</issuekey>
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