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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-64123] $elemMatch value predicate causes queries to accidentally fall back to the classic engine</title>
                <link>https://jira.mongodb.org/browse/SERVER-64123</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;When SBE was first implemented, it supported execution of $elemMatch value expressions. We still have plenty of code to handle translation of $elemMatch value to an SBE plan (&lt;a href=&quot;https://github.com/mongodb/mongo/blob/e27b00089fa1c46699b7a32b697304f7cd09386f/src/mongo/db/query/sbe_stage_builder_filter.cpp#L1297&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;see here&lt;/a&gt;).&lt;/p&gt;

&lt;p&gt;We subsequently made a change in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-57854&quot; title=&quot;Changed behavior in SBE when filter is {&amp;quot;&amp;quot;:null}&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-57854&quot;&gt;&lt;del&gt;SERVER-57854&lt;/del&gt;&lt;/a&gt; to fallback to the classic engine when there are predicates over empty field names. This is implemented by &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e27b00089fa1c46699b7a32b697304f7cd09386f/src/mongo/db/matcher/expression_parser.cpp#L105-L108&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;marking the query SBE-incompatible if any path predicate with an empty path is detected&lt;/a&gt;. This had the unintended side effect of preventing queries involving $elemMatch value from ever executing in SBE. The reason (as noted in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-59757&quot; title=&quot;Evaluate supporting queries with match expressions on empty field names in SBE&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-59757&quot;&gt;&lt;del&gt;SERVER-59757&lt;/del&gt;&lt;/a&gt;) is that the match expression parser &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e27b00089fa1c46699b7a32b697304f7cd09386f/src/mongo/db/matcher/expression_parser.cpp#L1301&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;internally creates predicates with empty field names&lt;/a&gt; as part of constructing the &lt;tt&gt;MatchExpression&lt;/tt&gt; tree to represent an $elemMatch value.&lt;/p&gt;

&lt;p&gt;This isn&apos;t technically a correctness bug, but it does leave the code base in a confusing state. The code appears to support $elemMatch value in SBE, but in fact it does not. The work for this ticket is to restore $elemMatch value support in SBE via the stage builders (or potentially delete the dead stage builder code?).&lt;/p&gt;</description>
                <environment></environment>
        <key id="1993250">SERVER-64123</key>
            <summary>$elemMatch value predicate causes queries to accidentally fall back to the classic engine</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</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="13201">Fixed</resolution>
                                        <assignee username="andrew.paroski@mongodb.com">Drew Paroski</assignee>
                                    <reporter username="david.storch@mongodb.com">David Storch</reporter>
                        <labels>
                            <label>pm2697-m2</label>
                            <label>quick-tech-debt</label>
                            <label>sbe</label>
                    </labels>
                <created>Wed, 2 Mar 2022 19:00:51 +0000</created>
                <updated>Sun, 29 Oct 2023 21:41:44 +0000</updated>
                            <resolved>Mon, 12 Sep 2022 16:04:41 +0000</resolved>
                                                    <fixVersion>6.2.0-rc0</fixVersion>
                                                        <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="4815470" author="xgen-internal-githook" created="Fri, 9 Sep 2022 19:13:06 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Drew Paroski&apos;, &apos;email&apos;: &apos;drew.paroski@mongodb.com&apos;, &apos;username&apos;: &apos;paroski&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-64123&quot; title=&quot;$elemMatch value predicate causes queries to accidentally fall back to the classic engine&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-64123&quot;&gt;&lt;del&gt;SERVER-64123&lt;/del&gt;&lt;/a&gt; Re-enable support for ElemMatchValueMatchExpressions in SBE&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/2f78c8ce493186984bd630b6e9678699d740a66c&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/2f78c8ce493186984bd630b6e9678699d740a66c&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="4664692" author="kyle.suarez" created="Thu, 7 Jul 2022 20:41:21 +0000"  >&lt;p&gt;CC &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=mickey.winters%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;mickey.winters@mongodb.com&quot;&gt;mickey.winters@mongodb.com&lt;/a&gt;, &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=jennifer.peshansky%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;jennifer.peshansky@mongodb.com&quot;&gt;jennifer.peshansky@mongodb.com&lt;/a&gt;&lt;/p&gt;</comment>
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            <issuekey id="1866413">SERVER-59757</issuekey>
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            <issuekey id="1866413">SERVER-59757</issuekey>
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