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    <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-54427] DISTINCT_SCAN can be used with compound index when filter is on non-multikey field</title>
                <link>https://jira.mongodb.org/browse/SERVER-54427</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;If you have an index on a:1, b:1 and distinct command or agg equivalent&lt;/p&gt;
&lt;p/&gt;
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					&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.foo.distinct(&quot;b&quot;, {a:1})&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;
&lt;p&gt;we use DISTINCT_SCAN &lt;b&gt;unless&lt;/b&gt; &lt;tt&gt;b&lt;/tt&gt; is multikey - and it&apos;s not clear why - it doesn&apos;t need IXSCAN nor FETCH (which is what it shows using).&lt;/p&gt;
</description>
                <environment></environment>
        <key id="1618011">SERVER-54427</key>
            <summary>DISTINCT_SCAN can be used with compound index when filter is on non-multikey field</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="4" iconUrl="https://jira.mongodb.org/images/icons/priorities/minor.svg">Minor - P4</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="asya.kamsky@mongodb.com">Asya Kamsky</reporter>
                        <labels>
                    </labels>
                <created>Tue, 9 Feb 2021 16:51:31 +0000</created>
                <updated>Sat, 11 Nov 2023 09:33:25 +0000</updated>
                                                                            <component>Querying</component>
                                        <votes>2</votes>
                                    <watches>10</watches>
                                                                                                                <comments>
                            <comment id="5873077" author="JIRAUSER1275786" created="Sat, 11 Nov 2023 09:33:25 +0000"  >&lt;p&gt;Currently I&apos;m using db.collection.distinct to get values from a multikey index. It&apos;s fast because it uses DISTINCT_SCAN. However, there&apos;s a 16mb document limit.&lt;/p&gt;

&lt;p&gt;db.collection.aggregate with $unwind and $group stage does a COLLSCAN or IXSCAN with FETCH (when forced with a hint) so it has to read the whole collection.&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;Are there any plans to implement the optimization to emit DISTINCT_SCAN for such cases?&#160;&lt;/p&gt;

&lt;p&gt;Is there currently another way to get distinct values from a multikey index with DISTINCT_SCAN beyond the 16m limit?&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;Thanks.&lt;/p&gt;</comment>
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                                                                <inwardlinks description="is duplicated by">
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            <issuekey id="340555">SERVER-27494</issuekey>
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