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    <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-58809] Optimize $unwind + $group _id, to avoid blocking/spilling</title>
                <link>https://jira.mongodb.org/browse/SERVER-58809</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Currently this is an antipattern:&lt;/p&gt;

&lt;p&gt;{$unwind: &quot;a&quot;}&lt;br/&gt;
{$match ...}&lt;br/&gt;
{$group: {_id: &quot;$_id&quot;, ...}}&lt;/p&gt;

&lt;p&gt;because $group is a blocking stage, and can spill if the data is big enough.  We recommend something like this instead:&lt;/p&gt;

&lt;p&gt;{$set: {a: {$filter ...}}}&lt;/p&gt;

&lt;p&gt;This performs better because it operates on one document at a time.&lt;/p&gt;

&lt;p&gt;But the first version is nicer in some ways:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;You can easily view intermediate results:
	&lt;ul&gt;
		&lt;li&gt;by commenting out stages,&lt;/li&gt;
		&lt;li&gt;or in Compass.&lt;/li&gt;
	&lt;/ul&gt;
	&lt;/li&gt;
	&lt;li&gt;You might not need to learn two versions of every operator ($match/$filter, $addFields/$map, $group/$reduce).&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;We could make it perform better by doing a streaming group (in this narrow case).&lt;/p&gt;

&lt;ul&gt;
	&lt;li&gt;Streaming $group is valid when documents are clustered by the group key.&lt;/li&gt;
	&lt;li&gt;Documents in a collection are clustered by _id (because we have a unique, non-multikey index on _id).&lt;/li&gt;
	&lt;li&gt;$unwind preserves this (if it unwinds one document at a time).&lt;/li&gt;
	&lt;li&gt;$match preserves this.&lt;/li&gt;
	&lt;li&gt;$project/$set can preserve this, depending on which paths they write.&lt;/li&gt;
&lt;/ul&gt;
</description>
                <environment></environment>
        <key id="1829700">SERVER-58809</key>
            <summary>Optimize $unwind + $group _id, to avoid blocking/spilling</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-execution">Backlog - Query Execution</assignee>
                                    <reporter username="david.percy@mongodb.com">David Percy</reporter>
                        <labels>
                    </labels>
                <created>Fri, 23 Jul 2021 17:57:13 +0000</created>
                <updated>Thu, 12 Jan 2023 01:03:39 +0000</updated>
                                                                                                <votes>0</votes>
                                    <watches>1</watches>
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