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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-62405] Partially-streaming compound $sort</title>
                <link>https://jira.mongodb.org/browse/SERVER-62405</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;In a pipeline like this:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{$sort: {x: 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;... // order-preserving stages&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;{$sort: {x: 1, y: 1}}&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
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&lt;p/&gt;
&lt;p&gt;we could optimize the second $sort, to take advantage of the fact that the input is already sorted on {x: 1}.  Any two documents with the same {x: 1} key are already next to each other, so whenever we see &apos;x&apos; increase, we can start returning results before asking for more input.  Depending on how big these runs of equal &apos;x&apos; are, this could save a lot of space, and maybe avoid spilling.&lt;/p&gt;

&lt;p&gt;Also, we only need to compare values of &apos;y&apos;, since we&apos;re only breaking ties within each run of equal &apos;x&apos; values.  Maybe that would be useful if &apos;x&apos; is large, or the overall document is small.&lt;/p&gt;

&lt;p&gt;This could be useful for cases like:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;{$densify ...} {$sort: {partitionField: 1, someOtherField: 1}}, because the output of $densify will be sorted by 
{partitionField: 1}
&lt;p&gt;.&lt;/p&gt;&lt;/li&gt;
	&lt;li&gt;{$unpackBucket ...} {$sort: {meta: 1, temperature: 1}}, because we could push down the sort on 
{meta: 1}
&lt;p&gt;.&lt;/p&gt;&lt;/li&gt;
	&lt;li&gt;{$unwind ...} {$sort ...} similar to $unpackBucket.&lt;/li&gt;
&lt;/ul&gt;
</description>
                <environment></environment>
        <key id="1959735">SERVER-62405</key>
            <summary>Partially-streaming compound $sort</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="david.percy@mongodb.com">David Percy</reporter>
                        <labels>
                    </labels>
                <created>Thu, 6 Jan 2022 20:47:51 +0000</created>
                <updated>Mon, 5 Dec 2022 20:23:56 +0000</updated>
                                                                                                <votes>0</votes>
                                    <watches>8</watches>
                                                                                                                <comments>
                            <comment id="5001435" author="david.percy" created="Tue, 22 Nov 2022 18:23:12 +0000"  >&lt;p&gt;Another way to think about this might be similar to recursive index navigation.  If you have an index {a: 1}, and you want to sort by {a: 1, b: 1}, we could have a nested loop, where we get unique &apos;a&apos; values and then do a separate blocking sort for each one.&lt;/p&gt;

&lt;p&gt;This would also work for cases like {$unpackBucket ...} {$sort: {meta: 1, temperature: 1}}, assuming you have an index on {meta: 1}.  Something like:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;distinct-scan {meta: 1}&lt;/li&gt;
	&lt;li&gt;for each unique value:
	&lt;ul&gt;
		&lt;li&gt;unpack&lt;/li&gt;
		&lt;li&gt;sort by temperature&lt;/li&gt;
	&lt;/ul&gt;
	&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;However this would not work for stages like $densify or $setWindowFields, which carry state between documents.&lt;/p&gt;</comment>
                            <comment id="4512654" author="pawel.terlecki" created="Thu, 28 Apr 2022 12:13:52 +0000"  >&lt;p&gt;I agree with &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=anton.korshunov%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;anton.korshunov@mongodb.com&quot;&gt;anton.korshunov@mongodb.com&lt;/a&gt;. A good idea but I would defer it to the new optimizer. Probably as a separate feature(s) as we also need corresponding support in SBE.&lt;/p&gt;</comment>
                            <comment id="4279451" author="david.percy" created="Thu, 6 Jan 2022 21:04:35 +0000"  >&lt;p&gt;This is kind of similar to streaming group, but doesn&apos;t run into the same problem where &quot;$group equality&quot; means something different than &quot;$sort equality&quot;.&lt;/p&gt;</comment>
                    </comments>
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                                            <outwardlinks description="related to">
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            <issuekey id="2118613">SERVER-69027</issuekey>
        </issuelink>
                            </outwardlinks>
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                                        <issuelink>
            <issuekey id="2014056">SERVER-65159</issuekey>
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