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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-16381] Index scans don&apos;t include the final &quot;end&quot; check within nscanned/keysExamined</title>
                <link>https://jira.mongodb.org/browse/SERVER-16381</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;When scanning an index we will check a key to confirm if we have hit the end. If we have hit the end, we will stop the scan but not include that final index entry scanned within the keysExamined counter.&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/master/src/mongo/db/exec/index_scan.cpp#L336&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;Leaving the index scan&lt;/a&gt;&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/blob/master/src/mongo/db/exec/index_scan.cpp#L342&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;Incrementing the keysExamined counter&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;Simple reproducer:&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; &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;for(i=0;i&amp;lt;111;i++){db.t2.insert({x:i})} &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.t2.ensureIndex({x: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.t2.find({x:{$in:[3,50,74,100]}}).explain(true) &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;
&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; &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;.... &quot;keysExamined&quot; : 7, &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;This would scan the keys at the x values of 3, 4, 50, 51, 74, 75, 100 and 101. A total of 8 keys, while the counter remains at 7&lt;/p&gt;</description>
                <environment></environment>
        <key id="172075">SERVER-16381</key>
            <summary>Index scans don&apos;t include the final &quot;end&quot; check within nscanned/keysExamined</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="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="david.storch@mongodb.com">David Storch</assignee>
                                    <reporter username="david.hows">David Hows</reporter>
                        <labels>
                            <label>ET</label>
                    </labels>
                <created>Tue, 2 Dec 2014 01:37:20 +0000</created>
                <updated>Wed, 28 Oct 2015 04:32:32 +0000</updated>
                            <resolved>Fri, 8 May 2015 14:12:10 +0000</resolved>
                                                    <fixVersion>3.1.3</fixVersion>
                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>5</watches>
                                                                                                                <comments>
                            <comment id="908629" author="xgen-internal-githook" created="Fri, 8 May 2015 14:09:44 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;dstorch&apos;, u&apos;name&apos;: u&apos;David Storch&apos;, u&apos;email&apos;: u&apos;david.storch@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-16381&quot; title=&quot;Index scans don&amp;#39;t include the final &amp;quot;end&amp;quot; check within nscanned/keysExamined&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-16381&quot;&gt;&lt;del&gt;SERVER-16381&lt;/del&gt;&lt;/a&gt; last key examined by IXSCAN stage should also count towards keysExamined&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/df7e94f086f23a4ee9a10679ce7c6cf2ecbb2720&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/df7e94f086f23a4ee9a10679ce7c6cf2ecbb2720&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="907656" author="david.storch" created="Thu, 7 May 2015 15:24:30 +0000"  >&lt;p&gt;We have decided to make this change now, as it falls out of a recent refactor to the IndexScan query execution stage (see &lt;a href=&quot;https://github.com/mongodb/mongo/commit/db59e0f31b12966f127bf39df5674e3239c57350&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;db59e0f31b12966&lt;/a&gt;).&lt;/p&gt;</comment>
                            <comment id="805501" author="david.storch" created="Thu, 15 Jan 2015 16:53:16 +0000"  >&lt;p&gt;In my opinion, this is works as designed. It is indeed the case that the IndexScan stage currently counts each retrieved key towards nscanned, as well as counting each skip &lt;em&gt;between&lt;/em&gt; ranges towards nscanned. The final end check is not counted.&lt;/p&gt;

&lt;p&gt;That said, the value of nscanned reflects the internal behavior of the index scanning logic. We do not promise to the user that there will be some invariant such as &quot;nscanned is equal to the number of matching keys&quot;, or even &quot;nscanned is equal to the number of matching keys plus the number of times that we check for the end of a contiguous scan range&quot;. The &lt;a href=&quot;http://docs.mongodb.org/manual/reference/method/cursor.explain/#explain.nscanned&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;docs&lt;/a&gt; are appropriately vague:&lt;/p&gt;
&lt;blockquote&gt;
&lt;p&gt;nscanned is the total number of index entries scanned (or documents for a collection scan).&lt;/p&gt;&lt;/blockquote&gt;

&lt;p&gt;The order of magnitude of the nscanned value is much more important than precisely what the number is. A user probably shouldn&apos;t have to care whether or not nscanned is 4 instead of 3, but should definitely be concerned if it is 100 instead of 10.&lt;/p&gt;

&lt;p&gt;Although I see the point that perhaps the final end check should also be counted, I am unsure if the argument is strong enough to require a behavior change in this area. Closing this ticket as Works as Designed for now, but of course happy to re-open for further debate or if there are any further concerns &lt;img class=&quot;emoticon&quot; src=&quot;https://jira.mongodb.org/images/icons/emoticons/smile.png&quot; height=&quot;16&quot; width=&quot;16&quot; align=&quot;absmiddle&quot; alt=&quot;&quot; border=&quot;0&quot;/&gt;&lt;/p&gt;

&lt;p&gt;Best,&lt;br/&gt;
Dave&lt;/p&gt;</comment>
                    </comments>
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            <issuekey id="190192">SERVER-17635</issuekey>
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