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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-41252] cannot get covered query with $expr with $eq only</title>
                <link>https://jira.mongodb.org/browse/SERVER-41252</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Hello,&lt;/p&gt;

&lt;p&gt;When using $expr, a query that should be covered by the index actually examine documents. &lt;/p&gt;

&lt;p&gt;Not only that, it searches for the values twice, once during the index scan and a second time during the fetch stage.&lt;/p&gt;</description>
                <environment></environment>
        <key id="772889">SERVER-41252</key>
            <summary>cannot get covered query with $expr with $eq only</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="1" iconUrl="https://jira.mongodb.org/images/icons/statuses/open.png" description="">Open</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-query-optimization">Backlog - Query Optimization</assignee>
                                    <reporter username="miguel.nieto@mongodb.com">Miguel Angel Nieto</reporter>
                        <labels>
                            <label>query-director-triage</label>
                            <label>query-product-scope-1</label>
                            <label>query-product-urgency-2</label>
                            <label>query-product-value-2</label>
                    </labels>
                <created>Tue, 21 May 2019 13:52:05 +0000</created>
                <updated>Tue, 6 Feb 2024 15:15:01 +0000</updated>
                                                                            <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>14</watches>
                                                                                                                <comments>
                            <comment id="5901028" author="JIRAUSER1270969" created="Thu, 23 Nov 2023 09:34:26 +0000"  >&lt;p&gt;This is very relevant to $lookup with pipelines, as they are forced to use $expr to reference let variables.&lt;/p&gt;</comment>
                            <comment id="5901026" author="JIRAUSER1270969" created="Thu, 23 Nov 2023 09:33:50 +0000"  >&lt;p&gt;if we are using $expr, we don&#8217;t produce covered projections.&#160;&lt;br/&gt;
Simple way to reproduce:&lt;/p&gt;

&lt;p&gt;If index on a is present,&lt;/p&gt;

&lt;p&gt;This will have fetch + projection simple&lt;br/&gt;
db.test.explain().aggregate([{$match: {$expr: {$eq:&#160;&lt;span class=&quot;error&quot;&gt;&amp;#91;&amp;quot;$a&amp;quot;, 1&amp;#93;&lt;/span&gt;}}}, {$project: {_id: 0, a: 1}}])&lt;/p&gt;

&lt;p&gt;But this will have projection covered:&lt;br/&gt;
db.test.explain().aggregate(&lt;a href=&quot;file://{$match: {a: 1}}, //{$project: {_id: 0, a: 1}}&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;\\{$match: {a: 1}}, \\{$project: {_id: 0, a: 1}}&lt;/a&gt;)&lt;/p&gt;</comment>
                            <comment id="2268742" author="craig.homa" created="Mon, 3 Jun 2019 15:51:44 +0000"  >&lt;p&gt;Scheduling to the backlog as this improvement will be somewhat time consuming for the team to implement until we invest in the internal representation of the expression tree. &lt;/p&gt;

&lt;p&gt;We believe this optimization would be incorrect if the index is multikey. &lt;/p&gt;</comment>
                            <comment id="2254050" author="asya" created="Tue, 21 May 2019 14:09:27 +0000"  >&lt;p&gt;The issue is that after rewriting $expr $eq portion, we leave that predicate in the tree and therefore need a FETCH to apply it to test.  This ticket is meant to track improving this (if possible).&lt;/p&gt;</comment>
                    </comments>
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            <issuekey id="2507293">SERVER-83529</issuekey>
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