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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>
    </build-info>


<item>
            <title>[SERVER-45011] Index used for whole IXSCAN plan is determined by index creation order</title>
                <link>https://jira.mongodb.org/browse/SERVER-45011</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;The following query is eligible for a whole IXSCAN solution on any index prefixed by &quot;a&quot;:&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.c.find({}, {&quot;b&quot;: 1, _id: 0}).sort({a: 1}).explain()&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;However, because we &quot;break&quot; in &lt;a href=&quot;https://github.com/mongodb/mongo/blob/72788deb99186ac2628604505147e68ac53cffc5/src/mongo/db/query/query_planner.cpp#L943-L957&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;this block&lt;/a&gt; as soon as any index which provides the sort is available, we miss out on covered plans. That is, we only consider the &quot;first&quot; whole IXSCAN solution we see.&lt;/p&gt;

&lt;p&gt;So, for example if I were to run:&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;&amp;gt; db.c.drop()&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;&amp;gt; db.c.createIndex({a: 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;&amp;gt; db.c.createIndex({a: 1, b: 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;&amp;gt; db.c.find({}, {&quot;b&quot;: 1, _id: 0}).sort({a: 1}).explain()&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;...&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;We would get a non-covered plan, because the &quot;a&quot; index comes first in the list of indexes in the catalog.&lt;/p&gt;

&lt;p&gt;But switch the order we create the indexes in:&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;&amp;gt; db.c.drop()&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;&amp;gt; db.c.createIndex({a: 1, b: 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;&amp;gt; db.c.createIndex({a: 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;&amp;gt; db.c.find({}, {&quot;b&quot;: 1, _id: 0}).sort({a: 1}).explain()&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;...&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 now we have a covered plan because the &quot;a, b&quot; index came first when looking for a whole IXSCAN solution.&lt;/p&gt;

&lt;p&gt;Note that this is a separate issue from &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-12769&quot; title=&quot;Queries that scan an entire index to provide a sort should be covered when possible&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-12769&quot;&gt;SERVER-12769&lt;/a&gt;, which is a problem when using a whole IXSCAN solution and the filter is non-empty. This ticket is relevant for cases where the filter is empty.&lt;/p&gt;

&lt;p&gt;Ideas to fix this:&lt;br/&gt;
 -Delete that &quot;break&quot; statement (and the similar one for reverse index scans). This will cause both plans to go through the multi planner and plan ranker. Since both plans will have the same output/work ratio, the covered one will be chosen by the plan ranker.&lt;br/&gt;
 -If we want the planner to only ever generate one whole IXSCAN solution, we can have it generate all of the possible ones and then see if any are covered, picking that one. This would save work of multi planning (which is basically useless for this case, since there&apos;s no filter and all of the indexes are equally selective) but add yet another special code path to the planner.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1040877">SERVER-45011</key>
            <summary>Index used for whole IXSCAN plan is determined by index creation order</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="13201">Fixed</resolution>
                                        <assignee username="james.wahlin@mongodb.com">James Wahlin</assignee>
                                    <reporter username="ian.boros@mongodb.com">Ian Boros</reporter>
                        <labels>
                            <label>greenerbuild</label>
                            <label>qopt-team</label>
                    </labels>
                <created>Fri, 6 Dec 2019 23:54:38 +0000</created>
                <updated>Sun, 29 Oct 2023 22:14:18 +0000</updated>
                            <resolved>Fri, 3 Jan 2020 18:07:40 +0000</resolved>
                                                    <fixVersion>4.3.3</fixVersion>
                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="2779005" author="christopher.harris" created="Sat, 1 Feb 2020 17:35:52 +0000"  >&lt;p&gt;I had a backlogged TODO item from last year to investigate why the &lt;tt&gt;rejectedPlans&lt;/tt&gt; array of &lt;tt&gt;.explain()&lt;/tt&gt; output was empty when an index was used to sort but there were also other indexes present that could do the same.&#160; I had observed that the plan depended on index creation order and I was suspecting that we might indeed be breaking out of a loop early.&#160; But I wasn&apos;t able to find such logic looking at master and we appeared to be looping on &lt;tt&gt;fullIndexList&lt;/tt&gt; so I was thoroughly confused.&#160; Then I decided to check previous versions of &lt;tt&gt;query_planner.cpp&lt;/tt&gt;... You can imagine what a pleasant surprise it was to discover the old &lt;tt&gt;break&lt;/tt&gt; statements and the fact that they had very recently been removed via this ticket!&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;Just swinging by to say thanks to &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=ian.boros&quot; class=&quot;user-hover&quot; rel=&quot;ian.boros&quot;&gt;ian.boros&lt;/a&gt; for filing and to &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=james.wahlin&quot; class=&quot;user-hover&quot; rel=&quot;james.wahlin&quot;&gt;james.wahlin&lt;/a&gt; for resolving!&lt;/p&gt;</comment>
                            <comment id="2702856" author="xgen-internal-githook" created="Fri, 3 Jan 2020 18:05:52 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;James Wahlin&apos;, &apos;email&apos;: &apos;james.wahlin@mongodb.com&apos;, &apos;username&apos;: &apos;jameswahlin&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-45011&quot; title=&quot;Index used for whole IXSCAN plan is determined by index creation order&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-45011&quot;&gt;&lt;del&gt;SERVER-45011&lt;/del&gt;&lt;/a&gt; Index used for whole IXSCAN plan is determined by index creation order&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/0faebb0f323afbfed89bb3d82f5c432ae6ec5232&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/0faebb0f323afbfed89bb3d82f5c432ae6ec5232&lt;/a&gt;&lt;/p&gt;</comment>
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            <issuekey id="111557">SERVER-12769</issuekey>
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