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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-11116] cursor limit has inconsistent effect on compound index scan count</title>
                <link>https://jira.mongodb.org/browse/SERVER-11116</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;(Filed on behalf of, originally discovered by &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=sasher%40nds.com&quot; class=&quot;user-hover&quot; rel=&quot;sasher@nds.com&quot;&gt;sasher@nds.com&lt;/a&gt;.)&lt;/p&gt;

&lt;p&gt;Revised description (without explain)&lt;/p&gt;

&lt;p&gt;When we query a collection with a compound index:&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;{a: 1, b: 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;and provide a limit on the result set, the number of index keys scanned in the compound index seems to vary widely depending on the range of values requested for the first key.&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;(assuming that limit is 200, index contains 2000 documents in range [1000, 3000) for every a)&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;find({a: {$in: [1], $b}: {$gte:1000, $lt: 3000}}) -&amp;gt; documents scanned will be equal to limit value + 1 (201)&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;find({a: {$in: [1, 2], $b}: {$gte:1000, $lt: 3000}}) -&amp;gt; documents scanned will be all documents in index matching filter + some constant (4002)&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;The inconsistency is in the way the limit count affects the first query but not the second where we are requesting 2 values for the first key instead of 1.&lt;br/&gt;
--------------&lt;/p&gt;


&lt;p&gt;Collection contains large number of documents with fields a and b (in addition to _id).  There is an index on &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;{a:1, b: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;A query with $in expression on a and sort of b with limit should use the index on a and b.  It does use the index (and optimization from &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-5063&quot; title=&quot;$in on first compound key element and limit efficiency&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-5063&quot;&gt;&lt;del&gt;SERVER-5063&lt;/del&gt;&lt;/a&gt;) when explain() is appended, however without explain even with hint it scans and sorts the whole collection.&lt;/p&gt;

&lt;p&gt;Here is output from the logs for query: &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.logs2.find({&quot;a&quot; : {$in : [10000, 2000]}, &quot;b&quot; : { $gt : 10000, $lt : 30000}}).sort({b:1}).limit(100).hint({a:1,b: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; with and without explain.&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;[conn244] query index.logs2 query: { query: { a: { $in: [ 10000.0, 2000.0 ] }, b: { $gt: 10000.0, $lt: 30000.0 } }, orderby: { b: 1.0 }, $hint: { a: 1.0, b: 1.0 }, $explain: true } ntoreturn:100 ntoskip:0 nscanned:101 scanAndOrder:1 keyUpdates:0 locks(micros) r:1314 nreturned:1 reslen:634 1ms&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;[conn244] query index.logs2 query: { query: { a: { $in: [ 10000.0, 2000.0 ] }, b: { $gt: 10000.0, $lt: 30000.0 } }, orderby: { b: 1.0 }, $hint: { a: 1.0, b: 1.0 } } ntoreturn:100 ntoskip:0 nscanned:15000 scanAndOrder:1 keyUpdates:0 locks(micros) r:91487 nreturned:100 reslen:4620 91ms&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;The only way to keep it from sorting the entire dataset without explain() is by using negative limit.&lt;/p&gt;</description>
                <environment></environment>
        <key id="93582">SERVER-11116</key>
            <summary>cursor limit has inconsistent effect on compound index scan count</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="hari.khalsa@10gen.com">hari.khalsa@10gen.com</assignee>
                                    <reporter username="sasher@nds.com">Shay Asher</reporter>
                        <labels>
                            <label>query_triage</label>
                    </labels>
                <created>Thu, 10 Oct 2013 00:42:58 +0000</created>
                <updated>Mon, 11 Jul 2016 17:37:59 +0000</updated>
                            <resolved>Wed, 15 Jan 2014 14:51:14 +0000</resolved>
                                    <version>2.4.5</version>
                    <version>2.5.3</version>
                                    <fixVersion>2.5.5</fixVersion>
                                    <component>Querying</component>
                                        <votes>7</votes>
                                    <watches>9</watches>
                                                                                                                <comments>
                            <comment id="482462" author="hari.khalsa@10gen.com" created="Wed, 15 Jan 2014 14:51:08 +0000"  >&lt;p&gt;This was fixed in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-1205&quot; title=&quot;$or sort does not use index ranges expressed in or clauses&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-1205&quot;&gt;&lt;del&gt;SERVER-1205&lt;/del&gt;&lt;/a&gt;.&lt;/p&gt;</comment>
                            <comment id="457526" author="asya" created="Sun, 17 Nov 2013 19:20:02 +0000"  >&lt;p&gt;Related to general case of $in and sort using compound index optimally.&lt;/p&gt;</comment>
                    </comments>
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                                        <issuelink>
            <issuekey id="18552">SERVER-3310</issuekey>
        </issuelink>
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                <attachments>
                            <attachment id="33382" name="server11116.js" size="1106" author="benety.goh@mongodb.com" created="Mon, 4 Nov 2013 17:54:13 +0000"/>
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