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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-20492] Incorrect index used for count query using wiredTiger that causes slow queries</title>
                <link>https://jira.mongodb.org/browse/SERVER-20492</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;In our slow query logs we can see that this query below is using the incorrect index which causes it to run slowly.&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;
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						&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;[conn3522] command haa_msgs.$cmd command: count { count: &quot;messages&quot;, query: { $or: [ { targetId: ObjectId(&apos;55fc54eecb05d8d2547334a1&apos;) }, { targetId: null } ], type: { $in: [ 2 ] }, createdAt: { $gt: new Date(0) } }, fields: null } planSummary: IXSCAN { createdAt: -1.0 } keyUpdates:0 writeConflicts:0 numYields:271 reslen:44 locks:{ Global: { acquireCount: { r: 544 } }, Database: { acquireCount: { r: 272 } }, Collection: { acquireCount: { r: 272 } } } 103ms&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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&lt;p/&gt;

&lt;p&gt;We have an index on this collection that I would expect to be used.  But the log implies it uses a different index to run the 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;{targetId:1, type:1, createdAt:-1}&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
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&lt;/table&gt;
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&lt;p/&gt;

&lt;p&gt;However, if I manually run a &lt;b&gt;find&lt;/b&gt; query using the same query conditions, the explain plan uses the expected index.&lt;/p&gt;

&lt;p&gt;This type of query has always behaved as expected in the past.  This problem occurs on version 3.0.4 and 3.0.6.&lt;/p&gt;

&lt;p&gt;Not sure what is going on here, but this is causing serious performance problems in our database.&lt;/p&gt;

</description>
                <environment></environment>
        <key id="230498">SERVER-20492</key>
            <summary>Incorrect index used for count query using wiredTiger that causes slow queries</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="4">Incomplete</resolution>
                                        <assignee username="-1">Unassigned</assignee>
                                    <reporter username="davidt">David Tsai</reporter>
                        <labels>
                    </labels>
                <created>Fri, 18 Sep 2015 18:41:40 +0000</created>
                <updated>Sat, 9 Jan 2016 13:21:22 +0000</updated>
                            <resolved>Sat, 9 Jan 2016 13:21:22 +0000</resolved>
                                    <version>3.0.4</version>
                    <version>3.0.6</version>
                                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>5</watches>
                                                                                                                <comments>
                            <comment id="1130285" author="ramon.fernandez" created="Sat, 9 Jan 2016 13:21:14 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=davidt&quot; class=&quot;user-hover&quot; rel=&quot;davidt&quot;&gt;davidt&lt;/a&gt;, we haven&apos;t heard back from you for some time so I&apos;m going to close this ticket. If this is still an issue for you please send the &lt;tt&gt;explain(true)&lt;/tt&gt; information requested above and we&apos;ll investigate further.&lt;/p&gt;

&lt;p&gt;Regards,&lt;br/&gt;
Ram&#243;n.&lt;/p&gt;</comment>
                            <comment id="1088203" author="ramon.fernandez" created="Fri, 13 Nov 2015 19:36:37 +0000"  >&lt;p&gt;Apologies for the long delay in responding &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=davidt&quot; class=&quot;user-hover&quot; rel=&quot;davidt&quot;&gt;davidt&lt;/a&gt;. If this is still an issue for you, can you please send the &lt;tt&gt;explain(true)&lt;/tt&gt; output for the operation that uses the wrong index, for the &lt;tt&gt;find()&lt;/tt&gt; command that uses the expected index, and for the original operation removing the &lt;tt&gt;targetId&lt;/tt&gt; portion of the query?&lt;/p&gt;

&lt;p&gt;Thanks,&lt;br/&gt;
Ram&#243;n.&lt;/p&gt;</comment>
                            <comment id="1037152" author="davidt" created="Fri, 18 Sep 2015 19:06:35 +0000"  >&lt;p&gt;&lt;br/&gt;
Experimenting a bit, it seems like removing the &lt;/p&gt;
{targetId:null}
&lt;p&gt; portion of the query, solves the problem.  As far I understand it, a null equality condition should work with compound indexes?&lt;/p&gt;</comment>
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