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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-18476] In-progress queries may return incorrect results if an index concurrently becomes multikey</title>
                <link>https://jira.mongodb.org/browse/SERVER-18476</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Query planning logic depends substantially on whether or not the set of available indices is multikey, and on which indexed paths are multikey. If this information changes due to a concurrent update or insert creating a new array path, in progress queries may be using a plan that is now invalid. One way to fix this would be to kill all in-progress queries for a particular collection when making a change to that collection&apos;s multikeyness metadata.&lt;/p&gt;</description>
                <environment></environment>
        <key id="204243">SERVER-18476</key>
            <summary>In-progress queries may return incorrect results if an index concurrently becomes multikey</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-execution">Backlog - Query Execution</assignee>
                                    <reporter username="dan@mongodb.com">Daniel Pasette</reporter>
                        <labels>
                            <label>storch</label>
                    </labels>
                <created>Thu, 14 May 2015 17:37:32 +0000</created>
                <updated>Tue, 6 Dec 2022 04:51:32 +0000</updated>
                                                                            <component>Querying</component>
                                        <votes>1</votes>
                                    <watches>15</watches>
                                                                                                                <comments>
                            <comment id="1915398" author="milkie" created="Fri, 8 Jun 2018 17:08:33 +0000"  >&lt;p&gt;The query system relies on the multikey flag to prohibit certain optimizations when scanning indexes that would result in &quot;actually incorrect&quot; results.  One example is from index_bounds_builder.cpp:&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;        // If the index is multikey, it doesn&apos;t matter what the tightness of the child is, we must&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;        // return INEXACT_FETCH. Consider a multikey index on &apos;a&apos; with document {a: [1, 2, 3]} and&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;        // query {a: {$ne: 3}}.  If we treated the bounds [MinKey, 3), (3, MaxKey] as exact, then we&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;        // would erroneously return the document!&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;</comment>
                            <comment id="1915379" author="asya" created="Fri, 8 Jun 2018 16:51:24 +0000"  >&lt;p&gt;Technically are the results actually incorrect or only incorrect if the operation was relying on &quot;point-in-time&quot; semantics?&lt;/p&gt;</comment>
                            <comment id="1153139" author="david.storch" created="Mon, 25 Jan 2016 20:30:05 +0000"  >&lt;p&gt;Moving this work from &quot;3.3 Required&quot; to &quot;Planning Bucket A&quot;, since the user impact of unexpected transitions to multikey is small and there are some tricky technical obstacles. Specifically, the technical obstacles are:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;A multi-update can cause a transition to multikey, thereby killing itself. We would have to create a special mechanism to prevent the multi-update plan itself from getting killed, rather than simply using Collection::invalidateAll() to kill all queries currently active in the collection.&lt;/li&gt;
	&lt;li&gt;Index catalog level operations (e.g. index drops) are currently the only callers of Collection::invalidateAll(), and these operations fail with a user assertion if there is an in-progress background index build. This is because Collection::invalidateAll() would kill the collection scan used to implement the background index build. It is not trivial to allow transitions to multikey to safely occur during a background index build, but it also is not ok for an update or insert operation to fail simply because it caused a multikeyness transition during a background index build.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;CC &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=milkie&quot; class=&quot;user-hover&quot; rel=&quot;milkie&quot;&gt;milkie&lt;/a&gt;&lt;/p&gt;</comment>
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                                        <issuelink>
            <issuekey id="30660">SERVER-4975</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="191003">SERVER-17678</issuekey>
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            <issuekey id="760762">SERVER-41058</issuekey>
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