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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>
    </build-info>


<item>
            <title>[SERVER-79636] equivalent() function for $expr is not collation-aware</title>
                <link>https://jira.mongodb.org/browse/SERVER-79636</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;The &lt;tt&gt;MatchExpression&lt;/tt&gt; interface offers &lt;a href=&quot;https://github.com/mongodb/mongo/blob/576aed4598810bd0766829be4ae20d8274ffa164/src/mongo/db/matcher/expression.h#L439&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;MatchExpression::equivalent()&lt;/tt&gt;&lt;/a&gt; which can be used to check whether two match expressions are the same. Consider the following two &lt;tt&gt;$expr&lt;/tt&gt; match expressions:&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;// Display the data in the collection.&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;MongoDB Enterprise &amp;gt; db.c.find()&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;64caa40c416866f24e97cc48&quot;), &quot;str&quot; : &quot;a&quot; }&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;64caa40e416866f24e97cc4a&quot;), &quot;str&quot; : &quot;A&quot; }&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;64caa410416866f24e97cc4c&quot;), &quot;str&quot; : &quot;b&quot; }&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;&amp;nbsp;&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;// Query using lowercase constant.&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;MongoDB Enterprise &amp;gt; db.c.find({$expr: {$eq: [&quot;$str&quot;, &quot;a&quot;]}}).collation({locale: &quot;en_US&quot;, strength: 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;   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;64caa40c416866f24e97cc48&quot;), &quot;str&quot; : &quot;a&quot; }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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						&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;64caa40e416866f24e97cc4a&quot;), &quot;str&quot; : &quot;A&quot; }&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;&amp;nbsp;&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;// Query using uppercase constant.&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;MongoDB Enterprise &amp;gt; db.c.find({$expr: {$eq: [&quot;$str&quot;, &quot;A&quot;]}}).collation({locale: &quot;en_US&quot;, strength: 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;   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;64caa40c416866f24e97cc48&quot;), &quot;str&quot; : &quot;a&quot; }&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;64caa40e416866f24e97cc4a&quot;), &quot;str&quot; : &quot;A&quot; }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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&lt;/table&gt;
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&lt;p/&gt;
&lt;p&gt;These two queries use the case-insensitive collation and therefore are identical in meaning. However, the implementation of &lt;tt&gt;ExprMatchExpression::equivalent()&lt;/tt&gt; is not collation-aware. Since we haven&apos;t implemented related ticket &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-30982&quot; title=&quot;Add mechanism to allow for checking Expression equivalency&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-30982&quot;&gt;SERVER-30982&lt;/a&gt; yet, &lt;tt&gt;ExprMatchExpression::equivalent()&lt;/tt&gt; currently &lt;a href=&quot;https://github.com/mongodb/mongo/blob/576aed4598810bd0766829be4ae20d8274ffa164/src/mongo/db/matcher/expression_expr.cpp#L111-L113&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;works by serializing both the left-hand side and right-hand side to a &lt;tt&gt;mongo::Value&lt;/tt&gt; representation and then comparing the resulting values with the simple collator&lt;/a&gt;. Because we&apos;re using the simple collator, these two expressions will erroneously be considered non-equivalent.&lt;/p&gt;

&lt;p&gt;This is not an issue which will result in a user facing bug as currently there is a stronger collation being used for comparison. Yet there is some potential that queries do miss out on a few optimizations due to a more strict comparison. The same also applies for the Hashing function from the Boolean simplification from &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-79018&quot; title=&quot;Implement MatchExpression hasher&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-79018&quot;&gt;&lt;del&gt;SERVER-79018&lt;/del&gt;&lt;/a&gt;. For the scope of this ticket the implementation of &lt;tt&gt;ExprMatchExpression::equivalent()&lt;/tt&gt; should respect comparisons with the collations in mind. This will have an effect on long-tailed customers.&lt;/p&gt;</description>
                <environment></environment>
        <key id="2407416">SERVER-79636</key>
            <summary>equivalent() function for $expr is not collation-aware</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="david.storch@mongodb.com">David Storch</reporter>
                        <labels>
                    </labels>
                <created>Wed, 2 Aug 2023 19:01:58 +0000</created>
                <updated>Thu, 1 Feb 2024 15:36:22 +0000</updated>
                                                                                                <votes>0</votes>
                                    <watches>7</watches>
                                                                                                                <comments>
                            <comment id="5767854" author="david.storch" created="Tue, 10 Oct 2023 14:59:01 +0000"  >&lt;p&gt;Do we have a route forward for how to fix the problem observed here? It&apos;s not clear to me that it&apos;s worth spending too much time on this unless we identify that it actually can cause a user-facing bug.&lt;/p&gt;</comment>
                            <comment id="5641032" author="JIRAUSER1261429" created="Thu, 17 Aug 2023 16:49:33 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-79018&quot; title=&quot;Implement MatchExpression hasher&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-79018&quot;&gt;&lt;del&gt;SERVER-79018&lt;/del&gt;&lt;/a&gt; implemented hasher for expressions in &lt;tt&gt;pipeline/expression_hasher.h&lt;/tt&gt;, so if the &lt;tt&gt;equivalent()&lt;/tt&gt; function is fixed the hasher should be changed accordingly as well.&lt;/p&gt;</comment>
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            <issuekey id="2393490">SERVER-79018</issuekey>
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