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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-53626] Minimize index scanning when retrieving distinct values grouped by more than one field</title>
                <link>https://jira.mongodb.org/browse/SERVER-53626</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;MongoDB &lt;tt&gt;4.2&lt;/tt&gt; introduced the ability to avoid full index scans for aggregation pipelines that include a specific type of &lt;tt&gt;$group&lt;/tt&gt; stage - those that logically request distinct information which can be obtained by scanning a single document.&#160; This was initially implemented via&#160;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-9507&quot; title=&quot;Optimize $sort+$group+$first pipeline to avoid full index scan&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-9507&quot;&gt;&lt;del&gt;SERVER-9507&lt;/del&gt;&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;As confirmed by this comment&#160;from a related enhancement in&#160;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-40090&quot; title=&quot;DISTINCT_SCAN in agg is only used when certain format of _id is specified&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-40090&quot;&gt;&lt;del&gt;SERVER-40090&lt;/del&gt;&lt;/a&gt;, this optimization currently only applies when the aggregation is grouping on a single field.&#160; We should consider expanding this optimization, or a similar one, to reduce index scanning when the &lt;tt&gt;_id&lt;/tt&gt; specified for the &lt;tt&gt;$group&lt;/tt&gt; is on multiple fields.&#160;&#160;&#160;&#160;&lt;/p&gt;</description>
                <environment></environment>
        <key id="1582836">SERVER-53626</key>
            <summary>Minimize index scanning when retrieving distinct values grouped by more than one field</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="christopher.harris@mongodb.com">Chris Harris</reporter>
                        <labels>
                            <label>indexv3</label>
                    </labels>
                <created>Wed, 6 Jan 2021 22:30:25 +0000</created>
                <updated>Sat, 21 Jan 2023 02:54:33 +0000</updated>
                                                                                                <votes>1</votes>
                                    <watches>11</watches>
                                                                                                                <comments>
                            <comment id="4149945" author="katherine.wu" created="Tue, 26 Oct 2021 21:28:36 +0000"  >&lt;p&gt;The proposed change in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-21992&quot; title=&quot;Inconsistent results when grouping by possibly missing values&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-21992&quot;&gt;SERVER-21992&lt;/a&gt; is different from &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-60977&quot; title=&quot;Make $group _id behavior with null and missing more consistent&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-60977&quot;&gt;&lt;del&gt;SERVER-60977&lt;/del&gt;&lt;/a&gt;, and would not help with this ticket. Unfortunately this optimization will be blocked &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-14832&quot; title=&quot;Distinct command result set may include or exclude null/undefined depending on presence of index&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-14832&quot;&gt;SERVER-14832&lt;/a&gt;, or on a new index format that distinguishes between null and missing. Moving back to open.&lt;/p&gt;

&lt;p&gt;WIP patch: &lt;a href=&quot;https://github.com/10gen/mongo/tree/kaywux/SERVER-53626&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/10gen/mongo/tree/kaywux/SERVER-53626&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="4146580" author="katherine.wu" created="Mon, 25 Oct 2021 20:17:31 +0000"  >&lt;p&gt;The distinct command (and our current version of the index) conflates the null and missing values. However, $group with a mult-field document _id currently distinguishes between these two values. Without &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-21992&quot; title=&quot;Inconsistent results when grouping by possibly missing values&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-21992&quot;&gt;SERVER-21992&lt;/a&gt;, this optimization would give us different results.&lt;/p&gt;</comment>
                            <comment id="3735660" author="JIRAUSER1258885" created="Fri, 23 Apr 2021 14:22:08 +0000"  >&lt;p&gt;As part of this ticket, it would be nice to handle a case of a key based on an array of a single index key:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&amp;gt; db.coll1.explain().aggregate({$group: {_id: [&lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;$a&quot;&lt;/span&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;]}})&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;... &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;stage&quot;&lt;/span&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; : &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;COLLSCAN&quot;&lt;/span&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; ...&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;the case with an object works fine:&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;&amp;gt; db.coll1.explain().aggregate({$group: {_id: {fieldA: &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;$a&quot;&lt;/span&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;}}})&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;... &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;stage&quot;&lt;/span&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; : &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;DISTINCT_SCAN&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;</comment>
                    </comments>
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                                        <issuelink>
            <issuekey id="244865">SERVER-21992</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="151423">SERVER-14832</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="1908757">SERVER-60977</issuekey>
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            <issuekey id="1687921">SERVER-56294</issuekey>
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