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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-28423] Log file says nreturned is 0 when it cannot be 0</title>
                <link>https://jira.mongodb.org/browse/SERVER-28423</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;I have this line in the log file as a slow query:&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;   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;2017-03-20T08:01:20.414+0000 I COMMAND [conn2424] command jidariDevDB.people_you_may_know command: find { find: &quot;people_you_may_know&quot;, filter: { User: ObjectId(&apos;57042936377c2e29146b6386&apos;), SuggestedUser: ObjectId(&apos;565d3e3b367c2ea030e016d5&apos;) }, projection: { Metadata: 1, Source: 1, Unread: 1, LastUpdated: 1, Points: 1 }, limit: 1, batchSize: 0, shardVersion: [ Timestamp 0|0, ObjectId(&apos;000000000000000000000000&apos;) ] } planSummary: IXSCAN { SuggestedUser: 1.0, User: 1.0 } cursorid:325486549227 keysExamined:1 docsExamined:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:0 reslen:234 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } protocol:op_command 886ms&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;As we can see the query is indexed and the keys exist and the document exists since &quot;keysExamined:1 docsExamined:1&quot;. &lt;/p&gt;

&lt;p&gt;My question is how nreturned is 0? Should not nreturned be 1?&lt;/p&gt;

&lt;p&gt;I have also verified this by:&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;   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.people_you_may_know.find({ User: ObjectId(&apos;57042936377c2e29146b6386&apos;), SuggestedUser: ObjectId(&apos;565d3e3b367c2ea030e016d5&apos;) }, { Metadata: 1, Source: 1, Unread: 1, LastUpdated: 1, Points: 1 }).explain(&quot;executionStats&quot;)&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;and got:&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;&quot;executionStats&quot; : {&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;		&quot;executionSuccess&quot; : true,&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;		&quot;nReturned&quot; : 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;		&quot;executionTimeMillis&quot; : 280,&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;		&quot;totalKeysExamined&quot; : 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;		&quot;totalDocsExamined&quot; : 1,&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;Why does the log file report that nreturned = 0?&lt;/p&gt;</description>
                <environment></environment>
        <key id="366841">SERVER-28423</key>
            <summary>Log file says nreturned is 0 when it cannot be 0</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="james.wahlin@mongodb.com">James Wahlin</assignee>
                                    <reporter username="anasser">Ahmed Nasser</reporter>
                        <labels>
                    </labels>
                <created>Wed, 22 Mar 2017 10:31:39 +0000</created>
                <updated>Mon, 17 Apr 2017 13:55:06 +0000</updated>
                            <resolved>Wed, 22 Mar 2017 15:18:34 +0000</resolved>
                                    <version>3.2.12</version>
                                                    <component>Logging</component>
                                        <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="1530353" author="anasser" created="Wed, 22 Mar 2017 15:17:07 +0000"  >&lt;p&gt;Thanks James. That was very helpful. Please feel free to close the issue.&lt;/p&gt;</comment>
                            <comment id="1530303" author="james.wahlin@10gen.com" created="Wed, 22 Mar 2017 14:41:56 +0000"  >&lt;p&gt;Hi Ahmed,&lt;/p&gt;

&lt;p&gt;The only way to determine the number of documents that an indexed query with a filter would iterate over would be to perform a count operation. This operation will have to iterate over the index entries so comes with a cost.&lt;/p&gt;

&lt;p&gt;If your find commands are always executed with a batchSize of 0, you can use the log entries for the subsequent getMore operations to capture nscanned/nreturned statistics.&lt;/p&gt;

&lt;p&gt;Thanks,&lt;br/&gt;
James &lt;/p&gt;</comment>
                            <comment id="1530239" author="anasser" created="Wed, 22 Mar 2017 13:51:33 +0000"  >&lt;p&gt;Any way to find out the number of documents in the result the cursor will iterate on?&lt;/p&gt;</comment>
                            <comment id="1530230" author="anasser" created="Wed, 22 Mar 2017 13:47:37 +0000"  >&lt;p&gt;Thank you James. This was enlightening. &lt;/p&gt;

&lt;p&gt;I believe this means that nreturned in the log file cannot be relied upon in determining the nscanned/nreturned ratio. Specially if the driver in use defaults batchSize to 0 as in my case.&lt;/p&gt;</comment>
                            <comment id="1530149" author="james.wahlin@10gen.com" created="Wed, 22 Mar 2017 12:31:10 +0000"  >&lt;p&gt;Hi Ahmed,&lt;/p&gt;

&lt;p&gt;The slow query you posted above shows that it was run with &lt;tt&gt;batchSize: 0&lt;/tt&gt;. When a query is run with a &lt;a href=&quot;https://docs.mongodb.com/manual/reference/method/cursor.batchSize/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;batchSize&lt;/a&gt; of 0, it is requesting that no results be returned. Only a cursorid is returned, which can then be used to iterate over the results. Given that &lt;tt&gt;nreturned:0&lt;/tt&gt; is what you would expect to see.&lt;/p&gt;

&lt;p&gt;Best regards,&lt;br/&gt;
James&lt;/p&gt;</comment>
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