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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-10279] a problem about covering a index</title>
                <link>https://jira.mongodb.org/browse/SERVER-10279</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;i read the document about index of Mongodb.&lt;br/&gt;
It says:&lt;br/&gt;
An index covers (page 324) a query, a covered query, when:&lt;br/&gt;
&#8226; all the fields in the query (page 42) are part of that index, and&lt;br/&gt;
&#8226; all the fields returned in the documents that match the query are in the same index&lt;/p&gt;

&lt;p&gt;but i ensure an index on one field, and can cover the index with the following query:&lt;br/&gt;
1)  db.sb.ensureIndex(&lt;/p&gt;
{&apos;wei&apos;:1}
&lt;p&gt;)&lt;br/&gt;
2)  db.sb.find(&lt;/p&gt;
{&apos;wei&apos;:&apos;www&apos;,&apos;name&apos;:&apos;wu&apos;}
&lt;p&gt;).explain()&lt;br/&gt;
{&lt;br/&gt;
	&quot;cursor&quot; : &quot;BtreeCursor wei_1&quot;,&lt;br/&gt;
	&quot;isMultiKey&quot; : false,&lt;br/&gt;
	&quot;n&quot; : 1,&lt;br/&gt;
	&quot;nscannedObjects&quot; : 1,&lt;br/&gt;
	&quot;nscanned&quot; : 1,&lt;br/&gt;
	&quot;nscannedObjectsAllPlans&quot; : 2,&lt;br/&gt;
	&quot;nscannedAllPlans&quot; : 2,&lt;br/&gt;
	&quot;scanAndOrder&quot; : false,&lt;br/&gt;
	&quot;indexOnly&quot; : false,&lt;br/&gt;
	&quot;nYields&quot; : 0,&lt;br/&gt;
	&quot;nChunkSkips&quot; : 0,&lt;br/&gt;
	&quot;millis&quot; : 0,&lt;br/&gt;
	&quot;indexBounds&quot; : &lt;/p&gt;
{
		&quot;wei&quot; : [
			[
				&quot;www&quot;,
				&quot;www&quot;
			]
		]
	}
&lt;p&gt;,&lt;br/&gt;
	&quot;server&quot; : &quot;srv03.lan.sankuai.com:27017&quot;&lt;br/&gt;
}&lt;br/&gt;
the explain show the index is covered.  &lt;br/&gt;
anyone tell me why?&lt;/p&gt;</description>
                <environment></environment>
        <key id="83022">SERVER-10279</key>
            <summary>a problem about covering a index</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</type>
                                            <priority id="2" iconUrl="https://jira.mongodb.org/images/icons/priorities/critical.svg">Critical - P2</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="-1">Unassigned</assignee>
                                    <reporter username="chaoren316699">Chao Wei</reporter>
                        <labels>
                    </labels>
                <created>Mon, 22 Jul 2013 03:13:57 +0000</created>
                <updated>Wed, 10 Dec 2014 23:10:26 +0000</updated>
                            <resolved>Tue, 23 Jul 2013 04:39:22 +0000</resolved>
                                                                                        <votes>0</votes>
                                    <watches>3</watches>
                                                                                                                <comments>
                            <comment id="386749" author="dan@10gen.com" created="Tue, 23 Jul 2013 04:39:22 +0000"  >&lt;p&gt;This explain output does not show that the query is covered.&lt;/p&gt;

&lt;p&gt;The &quot;indexOnly&quot; field indicates that the query is satisfied from the index alone.&lt;/p&gt;

&lt;p&gt;See the documentation of the explain field here: &lt;a href=&quot;http://docs.mongodb.org/manual/reference/method/cursor.explain/#explain.indexOnly&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;http://docs.mongodb.org/manual/reference/method/cursor.explain/#explain.indexOnly&lt;/a&gt;&lt;/p&gt;</comment>
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