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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-31898] Improve sort analysis to allow multikey indexes to provide sorts when possible</title>
                <link>https://jira.mongodb.org/browse/SERVER-31898</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Due to the changes in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-19402&quot; title=&quot;Change semantics of sorting by array fields in find and aggregate&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-19402&quot;&gt;&lt;del&gt;SERVER-19402&lt;/del&gt;&lt;/a&gt;, it is no longer correct for a multikey index scan to provide a non-blocking sort. However, there are some special cases it which it is correct to do so. Specifically, it is correct when:&lt;/p&gt;
&lt;ol&gt;
	&lt;li&gt;The index bounds for all fields of the sort pattern are [MinKey, MaxKey].&lt;/li&gt;
	&lt;li&gt;There are no bounds over a multikey field which shares a path prefix with the sort pattern.&lt;/li&gt;
&lt;/ol&gt;


&lt;p&gt;This ticket tracks the work to improve the query planner so that it can generate non-blocking sorts with multikey indexes when possible.&lt;/p&gt;

&lt;p&gt;To explain constraint #1, consider the case when you have index {myArray: 1} and you issue the following query:&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.c.find({myArray: {$gte: 5}}).sort({myArray: 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;{ &quot;_id&quot; : ObjectId(&quot;5a04af3cb778d4774430038b&quot;), &quot;myArray&quot; : [ 1, 6, 7 ] }&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;{ &quot;_id&quot; : ObjectId(&quot;5a04af35b778d4774430038a&quot;), &quot;myArray&quot; : [ 3, 4, 5 ] }&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;If we were to use an index scan with bounds [5, Infinity] to answer this query, then these documents would sort incorrectly---the second document would use 5 as its sort key and the first document would use 6.&lt;/p&gt;

&lt;p&gt;Constraint #2 is a bit more subtle. Suppose we have an index {&quot;a.b&quot;: 1, &quot;a.c&quot;: 1} and the query&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.c.find({&quot;a.b&quot;: 0}).sort({&quot;a.c&quot;: 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;{ &quot;_id&quot; : 1, &quot;a&quot; : [ { &quot;b&quot; : 0, &quot;c&quot; : 1 }, { &quot;b&quot; : 1, &quot;c&quot; : -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;   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;_id&quot; : 0, &quot;a&quot; : [ { &quot;b&quot; : 1, &quot;c&quot; : 1 }, { &quot;b&quot; : 0, &quot;c&quot; : 0 } ] }&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 correct sort order is _id:1 and then _id:0, because c:-1 is smaller then c:0. However, if we were to use a multikey index scan with bounds on &quot;a.b&quot;, we would end up using c:0 as a sort key for the document with _id:0 and c:1 as a sort key for the document with _id:1. This would sort the documents in the incorrect order. The problem is that the bounds on &quot;a.b&quot; constrain which keys along the path &quot;a.c&quot; we scan, even though the index bounds for &quot;a.c&quot; are [MinKey, MaxKey].&lt;/p&gt;</description>
                <environment></environment>
        <key id="456863">SERVER-31898</key>
            <summary>Improve sort analysis to allow multikey indexes to provide sorts when possible</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</type>
                                            <priority id="4" iconUrl="https://jira.mongodb.org/images/icons/priorities/minor.svg">Minor - P4</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="13201">Fixed</resolution>
                                        <assignee username="ian.boros@mongodb.com">Ian Boros</assignee>
                                    <reporter username="david.storch@mongodb.com">David Storch</reporter>
                        <labels>
                    </labels>
                <created>Thu, 9 Nov 2017 19:53:13 +0000</created>
                <updated>Mon, 30 Oct 2023 23:10:39 +0000</updated>
                            <resolved>Tue, 28 Jan 2020 20:27:53 +0000</resolved>
                                                    <fixVersion>4.3.4</fixVersion>
                                    <component>Querying</component>
                                        <votes>2</votes>
                                    <watches>18</watches>
                                                                                                                <comments>
                            <comment id="2768932" author="xgen-internal-githook" created="Tue, 28 Jan 2020 19:12:05 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;email&apos;: &apos;ian.boros@mongodb.com&apos;, &apos;username&apos;: &apos;puppyofkosh&apos;, &apos;name&apos;: &apos;Ian Boros&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-31898&quot; title=&quot;Improve sort analysis to allow multikey indexes to provide sorts when possible&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-31898&quot;&gt;&lt;del&gt;SERVER-31898&lt;/del&gt;&lt;/a&gt; Allow multikey indexes to provide sorts in certain cases&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/ccc13ef0b7ce0f908479522660a54dcbd142f7a1&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/ccc13ef0b7ce0f908479522660a54dcbd142f7a1&lt;/a&gt;&lt;/p&gt;</comment>
                    </comments>
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                                                                <inwardlinks description="is duplicated by">
                                        <issuelink>
            <issuekey id="736677">SERVER-40599</issuekey>
        </issuelink>
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                            <issuelinktype id="10012">
                    <name>Related</name>
                                                                <inwardlinks description="is related to">
                                        <issuelink>
            <issuekey id="499717">SERVER-33387</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="1118973">SERVER-45823</issuekey>
        </issuelink>
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