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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-27744] merge sequential $project stages and $addFields stages when appropriate</title>
                <link>https://jira.mongodb.org/browse/SERVER-27744</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Consider&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{&quot;$project&quot;: {&quot;__joinedPipeline_1_c&quot;:&quot;$c&quot;,&quot;__joinedPipeline_1_e&quot;:&quot;$e&quot;,&quot;_id&quot;:&quot;$__joined_bar._id&quot;,&quot;c&quot;:&quot;$__joined_bar.c&quot;}},&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;{&quot;$project&quot;: {&quot;__joinedPipeline_1_c&quot;:1,&quot;__joinedPipeline_1_e&quot;:1,&quot;_id&quot;:1,&quot;bar_DOT__id&quot;:&quot;$_id&quot;,&quot;bar_DOT_c&quot;:&quot;$c&quot;,&quot;c&quot;:1}},&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{&quot;$project&quot;:{&quot;bar_DOT_c&quot;:&quot;$c&quot;,&quot;foo_DOT_e&quot;:&quot;$__joinedPipeline_1_e&quot;}}&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;&lt;br/&gt;
The pipeline can be consolidated into a single &lt;tt&gt;$project&lt;/tt&gt; stage:&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{&quot;$project&quot;:{&quot;bar_DOT_c&quot;:&quot;$__joined_bar.c&quot;,&quot;foo_DOT_e&quot;:&quot;$e&quot;}}&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;&lt;/p&gt;</description>
                <environment></environment>
        <key id="346826">SERVER-27744</key>
            <summary>merge sequential $project stages and $addFields stages when appropriate</summary>
                <type id="2" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14711&amp;avatarType=issuetype">New Feature</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="adinoyi.omuya@mongodb.com">Adinoyi Omuya</reporter>
                        <labels>
                            <label>neweng</label>
                            <label>optimization</label>
                            <label>patrick</label>
                    </labels>
                <created>Wed, 18 Jan 2017 22:50:39 +0000</created>
                <updated>Tue, 30 Jan 2024 14:37:17 +0000</updated>
                                                                            <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>12</watches>
                                                                                                                <comments>
                            <comment id="2025821" author="asya" created="Sat, 6 Oct 2018 15:07:29 +0000"  >&lt;p&gt;Here are detailed numbers, on same collection, same plan, doing $count following single $project with 7 fields removed vs 7 $projects each with single field removed (after several warm-up runs):&lt;/p&gt;

&lt;p&gt;Single $project: 57ms to 66ms over four runs.&lt;br/&gt;
Seven $projects: 112ms to 119ms over four runs.&lt;/p&gt;</comment>
                            <comment id="2019201" author="asya" created="Mon, 1 Oct 2018 15:30:00 +0000"  >&lt;p&gt;I just did some quick benchmarking and on an &quot;extreme&quot; case - only N sequential project or sequential addFields stages, compared with a single same stage, when you have about seven stages, the execution time on the server roughly doubles.  So this is worthwhile an effort.&lt;/p&gt;

</comment>
                            <comment id="1858444" author="asya" created="Sat, 7 Apr 2018 16:56:00 +0000"  >&lt;p&gt;Yes the example given is coalesceable, however, the three project example in the description gives result with &amp;#95;id field equal to original &amp;#95;&amp;#95;joined_bar._id and in the second single stage it&apos;s the original field.  I think the single combined stage has to be corrected to:&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;
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					&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;{&quot;$project&quot;:{_id:&quot;$__joined_bar._id&quot;,&quot;bar_DOT_c&quot;:&quot;$__joined_bar.c&quot;,&quot;foo_DOT_e&quot;:&quot;$e&quot;}}&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;As far as inclusion and exclusion combining, while this example you gave with all inclusion project is combinable, I was pointing out that mixing $projects with inclusions and $projects with exclusions is not really possible.&lt;/p&gt;

&lt;p&gt;The tricky part will be &quot;when appropriate&quot; qualification.  &lt;img class=&quot;emoticon&quot; src=&quot;https://jira.mongodb.org/images/icons/emoticons/smile.png&quot; height=&quot;16&quot; width=&quot;16&quot; align=&quot;absmiddle&quot; alt=&quot;&quot; border=&quot;0&quot;/&gt;&lt;/p&gt;</comment>
                            <comment id="1747013" author="adinoyi.omuya@10gen.com" created="Fri, 8 Dec 2017 17:07:53 +0000"  >&lt;p&gt;Do you mean that since the expected coalesced $project excludes _id (an exclusion), the server won&apos;t be able to coalesce the 3 stages? If so, why? Why would multiple {{$project}}s that contain inclusion/exclusion directives preclude coalescing?&lt;/p&gt;</comment>
                            <comment id="1678123" author="adinoyi.omuya@10gen.com" created="Wed, 20 Sep 2017 20:52:37 +0000"  >&lt;p&gt;I&apos;m not following Asya, I think the example I included in the description should be coalesce-able. Do you agree? If not, why won&apos;t it be possible?&lt;/p&gt;</comment>
                            <comment id="1572731" author="asya" created="Tue, 16 May 2017 20:22:06 +0000"  >&lt;p&gt;I believe only all inclusion or all exclusion stages can be coalesced (i.e. we can&apos;t coalesce an exclusion project with an $addFields or inclusion $project).&lt;/p&gt;</comment>
                            <comment id="1560566" author="asya" created="Fri, 28 Apr 2017 19:16:56 +0000"  >&lt;p&gt;And answering my own question, after testing for &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-29010&quot; title=&quot;Coalesce adjacent exclusion $project stages and non-overlapping $addFields stages&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-29010&quot;&gt;&lt;del&gt;SERVER-29010&lt;/del&gt;&lt;/a&gt; coalescing these will make the pipeline run faster.&lt;/p&gt;

&lt;p&gt;In addition, coalescing adjacent $addFields stages will also improve performance - this ticket can track both.&lt;/p&gt;</comment>
                            <comment id="1486414" author="asya" created="Thu, 26 Jan 2017 23:50:01 +0000"  >&lt;p&gt;it&apos;ll make the pipeline shorter - would this have any measurable performance improvement?&lt;/p&gt;</comment>
                    </comments>
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