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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>[DOCS-976] Improve description of nscanned and nscannedObjects</title>
                <link>https://jira.mongodb.org/browse/DOCS-976</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;p&gt;See: &lt;a href=&quot;http://docs.mongodb.org/manual/reference/explain/#explain-output-fields-core&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;http://docs.mongodb.org/manual/reference/explain/#explain-output-fields-core&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;Is &lt;tt&gt;nscanned&lt;/tt&gt; always equal to the number of documents scanned on disk (i.e. &lt;tt&gt;nscannedObjects&lt;/tt&gt;) plus any additional index entries scanned?&lt;/p&gt;

&lt;p&gt;Can we clarify what &quot;if the index is a covered index&quot; means? &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=william.zola%4010gen.com&quot; class=&quot;user-hover&quot; rel=&quot;william.zola@10gen.com&quot;&gt;william.zola@10gen.com&lt;/a&gt; said it likely means that the index is used for the entire query predicate (i.e. all fields in our criteria are also in the index).&lt;/p&gt;

&lt;p&gt;If it helps, here&apos;s some discussion we had about it to provide context to my confusion:&lt;/p&gt;

&lt;blockquote&gt;
&lt;p&gt;Jeremy: yeah, covered index means nscannedObjects is 0, since we look at nothing on disk&lt;br/&gt;
Jeremy: saying &quot;the value may be lower than nscanned&quot; isn&apos;t as clear as saying &quot;it should be 0&quot; though, unless there&apos;s a case i&apos;m missing where it may be covered but still not 0&lt;br/&gt;
William: I agree, docs could be better on query engine.&lt;br/&gt;
William: defining what they mean by a &apos;covered index&apos; would be a start.  Since I don&apos;t think they&apos;re using that term to mean &apos;covered query&apos;, which is a different beast altogether&lt;br/&gt;
Jeremy: perhaps they just mean a query that uses an index&lt;br/&gt;
Jeremy: i&apos;ll open a docs ticket for this&lt;br/&gt;
William: What they mean is a query where the index can satisfy the entire query predicate.&lt;br/&gt;
Jeremy: oh, so the entire matched set comes from the index&lt;br/&gt;
William: If the query condition includes fields not present in the index =&amp;gt; nscannedObjects will be non-zero&lt;br/&gt;
William: (since the query engine will have to fetch the document &amp;amp; examine it to see if it matches the condition)&lt;br/&gt;
William: @Jeremy: right&lt;br/&gt;
Jeremy: and if the query condition includes fields only in the index, but it&apos;s not index-only for the projected fields, nscannedObjects will be equal to n&lt;br/&gt;
William: I think this also applies if you sort by a non-indexed field, but I wouldn&apos;t bet the rent on it.&lt;br/&gt;
William: I believe that to be correct as well.&lt;/p&gt;&lt;/blockquote&gt;</description>
                <environment></environment>
        <key id="61708">DOCS-976</key>
            <summary>Improve description of nscanned and nscannedObjects</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</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="5">Cannot Reproduce</resolution>
                                        <assignee username="kay.kim@mongodb.com">Kay Kim</assignee>
                                    <reporter username="jmikola@mongodb.com">Jeremy Mikola</reporter>
                        <labels>
                    </labels>
                <created>Fri, 11 Jan 2013 17:35:53 +0000</created>
                <updated>Mon, 30 Oct 2023 22:33:31 +0000</updated>
                            <resolved>Wed, 7 Aug 2013 14:24:33 +0000</resolved>
                                                    <fixVersion>Server_Docs_20231030</fixVersion>
                                    <component>manual</component>
                        <due></due>
                            <votes>0</votes>
                                    <watches>2</watches>
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