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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-53297] Return sorted query results in random order amongst sets of documents with equal sort keys</title>
                <link>https://jira.mongodb.org/browse/SERVER-53297</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;&lt;b&gt;Motivation&lt;/b&gt;. Application developers sometimes accidentally rely on the order of results returned by sort constructs such as &lt;a href=&quot;https://docs.mongodb.com/manual/reference/operator/aggregation/sort/index.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;$sort aggregation stage&lt;/a&gt; or &lt;a href=&quot;https://docs.mongodb.com/manual/reference/method/cursor.sort/index.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;sort() method&lt;/a&gt; in addition to what is guaranteed by the sort construct. That may happen because in some queries, circumstances, and versions of the server there could be an apparent order of the results that is consistent between queries. However, that order is not guaranteed by the server and can cause system correctness problems, such as HELP-20458, when the server configuration changes (such as when a single replica set is transformed to a sharded cluster) or the server evolves.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;Proposed solution&lt;/b&gt;. To increase a probability of detecting this design mistake earlier in the lifecycle of the application systems, randomize the order of the results produced by sort constructs. That is, if the values of fields to sort by are equal then the relative order of such documents should be random (stochastic) and not persistent between queries.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;Example&lt;/b&gt;&lt;/p&gt;

&lt;p&gt;Given the collection &lt;em&gt;coll&lt;/em&gt;:&lt;br/&gt;
 &#160;&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;   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;db.coll.insert({a:1, b:1}, {a:2, b:2}, {a:1, b:3}, {a:2, b:4})&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;&#160;&lt;br/&gt;
 When issuing a command&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;   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;db.coll.aggregate([{$sort: {a: 1}}])&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;ideally, a probability of getting &lt;b&gt;Result A&lt;/b&gt; or &lt;b&gt;Result B&lt;/b&gt; should be the same and equal to 0.25.&lt;/p&gt;

&lt;p&gt;Result A:&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;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;_id&quot; : ObjectId(&quot;5fd0a1937a7cb559d4cff62b&quot;), &quot;a&quot; : 1, &quot;b&quot; : 1 }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&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;   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;5fd0a1937a7cb559d4cff62d&quot;), &quot;a&quot; : 1, &quot;b&quot; : 3 }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&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;   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;5fd0a1937a7cb559d4cff62c&quot;), &quot;a&quot; : 2, &quot;b&quot; : 2 }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&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-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;5fd0a1937a7cb559d4cff62e&quot;), &quot;a&quot; : 2, &quot;b&quot; : 4 }&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;Result B:&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;{ &quot;_id&quot; : ObjectId(&quot;5fd0a1937a7cb559d4cff62d&quot;), &quot;a&quot; : 1, &quot;b&quot; : 3 }&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;5fd0a1937a7cb559d4cff62b&quot;), &quot;a&quot; : 1, &quot;b&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; : ObjectId(&quot;5fd0a1937a7cb559d4cff62c&quot;), &quot;a&quot; : 2, &quot;b&quot; : 2 }&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;5fd0a1937a7cb559d4cff62e&quot;), &quot;a&quot; : 2, &quot;b&quot; : 4 }&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;&lt;b&gt;Design considerations&lt;/b&gt;&lt;/p&gt;

&lt;p&gt;However, a perfect randomization (that produces any possible permutation of results with equal probability) is not required, just &#8220;random enough&#8221; to achieve the goal. Also, if necessary, trade-offs should be made to favour minimization of resource usage overhead of the feature instead of perfect randomization.&lt;/p&gt;

&lt;p&gt;This feature could have a toggle for turning it off at the server/query level.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1563704">SERVER-53297</key>
            <summary>Return sorted query results in random order amongst sets of documents with equal sort keys</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="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="12300">Won&apos;t Do</resolution>
                                        <assignee username="backlog-query-execution">Backlog - Query Execution</assignee>
                                    <reporter username="mindaugas.malinauskas@mongodb.com">Mindaugas Malinauskas</reporter>
                        <labels>
                    </labels>
                <created>Wed, 9 Dec 2020 11:14:03 +0000</created>
                <updated>Tue, 6 Dec 2022 01:41:58 +0000</updated>
                            <resolved>Tue, 5 Jan 2021 16:53:58 +0000</resolved>
                                                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="3549049" author="david.storch" created="Tue, 5 Jan 2021 16:56:40 +0000"  >&lt;p&gt;We decided not to do this because although it would be useful to help customers find bugs in their applications, it would be wasted work for the server to perform. Therefore, it is the kind of behavior that we would only want to introduce in debug builds. However, debug builds are only used for our internal testing and are not shipped to customers. This in turn means customers would not be able to take advantage of this behavior to find bugs in their application.&lt;/p&gt;</comment>
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            <issuekey id="1510789">SERVER-51498</issuekey>
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