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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-15528] Distinct queries can scan many index keys without yielding read lock</title>
                <link>https://jira.mongodb.org/browse/SERVER-15528</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;For certain distinct queries, the distinct scan query stage can scan many index keys between returning results.  Distinct queries that generate inexact bounds and encounter non-matching index entries (and thus generate a call to skip() on the underlying index cursor) are affected.  As a result, the distinct query can scan a large index range without yielding its read lock.&lt;/p&gt;

&lt;p&gt;To reproduce, run the following shell snippet.  The distinct query on the third line will scan the complete {a: 1, b: 1} index without yielding.&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: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;for&lt;/span&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; (i=0; i&amp;lt;1000000; i++) { db.foo.insert({a: i, b: 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;db.foo.ensureIndex({a: 1, b: 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;   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(&lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;a&quot;&lt;/span&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;, {a: {$ne: 0}, b: 0});&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;Regression introduced in 2.6.0.&lt;/p&gt;</description>
                <environment></environment>
        <key id="161860">SERVER-15528</key>
            <summary>Distinct queries can scan many index keys without yielding read lock</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="9">Done</resolution>
                                        <assignee username="rassi">J Rassi</assignee>
                                    <reporter username="rassi">J Rassi</reporter>
                        <labels>
                    </labels>
                <created>Fri, 3 Oct 2014 18:21:24 +0000</created>
                <updated>Mon, 11 Jul 2016 17:19:06 +0000</updated>
                            <resolved>Wed, 15 Oct 2014 04:13:57 +0000</resolved>
                                    <version>2.6.0</version>
                                    <fixVersion>2.6.6</fixVersion>
                    <fixVersion>2.7.8</fixVersion>
                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="741363" author="xgen-internal-githook" created="Wed, 15 Oct 2014 04:13:34 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;jrassi&apos;, u&apos;name&apos;: u&apos;Jason Rassi&apos;, u&apos;email&apos;: u&apos;rassi@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-15528&quot; title=&quot;Distinct queries can scan many index keys without yielding read lock&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-15528&quot;&gt;&lt;del&gt;SERVER-15528&lt;/del&gt;&lt;/a&gt; Refactor DistinctScan to do less in each work() call&lt;/p&gt;

&lt;p&gt;(cherry picked from commit e0f47214d06738f14d0d0d731235a9f665a6b81f)&lt;br/&gt;
Branch: v2.6&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/638f6b7c9b02c30b53ac9e54a47466cb8dee194f&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/638f6b7c9b02c30b53ac9e54a47466cb8dee194f&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="741360" author="xgen-internal-githook" created="Wed, 15 Oct 2014 04:06:36 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;jrassi&apos;, u&apos;name&apos;: u&apos;Jason Rassi&apos;, u&apos;email&apos;: u&apos;rassi@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-15528&quot; title=&quot;Distinct queries can scan many index keys without yielding read lock&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-15528&quot;&gt;&lt;del&gt;SERVER-15528&lt;/del&gt;&lt;/a&gt; Refactor DistinctScan to do less in each work() call&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/e0f47214d06738f14d0d0d731235a9f665a6b81f&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/e0f47214d06738f14d0d0d731235a9f665a6b81f&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="733807" author="rassi@10gen.com" created="Fri, 3 Oct 2014 18:23:30 +0000"  >&lt;p&gt;This ticket describes the same fundamental issue as &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-15152&quot; title=&quot;When evaluating plans, some index candidates cause complete index scan&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-15152&quot;&gt;&lt;del&gt;SERVER-15152&lt;/del&gt;&lt;/a&gt;, except for the distinct scan stage instead of the index scan stage.&lt;/p&gt;</comment>
                    </comments>
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            <issuekey id="156517">SERVER-15152</issuekey>
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