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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-15636] Query Optimizer Index Scans when first predicate is unbounded.</title>
                <link>https://jira.mongodb.org/browse/SERVER-15636</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Imagine you have fields A &amp;amp; B and a compound index defined as &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;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;

&lt;p&gt;Now imagine a 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; find( { B : value }).sort(A) &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;

&lt;p&gt;In 2.4 it appears we would jump to all values of A and within each subset of A skip to entries where B=&quot;value&quot; in the index. Leading to a smaller nscannedObjects() value in the query plan.&lt;/p&gt;

&lt;p&gt;As of 2.6 it appears indexed plans where the first field in the index is unconstrained are not considered.  Which means we end up not using plans in 2.6 that would sometimes get used in 2.4&lt;/p&gt;

&lt;p&gt;In this particular case, we end up essentially doing a full index scan. Which depending on data distribution, will most likely be slower than in 2.4.&lt;/p&gt;</description>
                <environment></environment>
        <key id="163407">SERVER-15636</key>
            <summary>Query Optimizer Index Scans when first predicate is unbounded.</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</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="3">Duplicate</resolution>
                                        <assignee username="james.wahlin@mongodb.com">James Wahlin</assignee>
                                    <reporter username="osmar.olivo">Osmar Olivo</reporter>
                        <labels>
                            <label>2426</label>
                    </labels>
                <created>Mon, 13 Oct 2014 21:49:59 +0000</created>
                <updated>Tue, 19 Oct 2021 14:14:15 +0000</updated>
                            <resolved>Tue, 19 Oct 2021 14:14:15 +0000</resolved>
                                    <version>2.6.0</version>
                                                    <component>Querying</component>
                                        <votes>1</votes>
                                    <watches>12</watches>
                                                                                                                <comments>
                            <comment id="4132306" author="james.wahlin@10gen.com" created="Tue, 19 Oct 2021 14:13:59 +0000"  >&lt;p&gt;Closing as a duplicate of &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-13197&quot; title=&quot;Tighten index bounds and allow compound index to be chosen when predicate on leading field is not provided&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-13197&quot;&gt;SERVER-13197&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;It is worth noting for anyone following that we generally recommend ordering index keys in the following priority:&lt;br/&gt;
1) Fields used for equality matches&lt;br/&gt;
2) Fields used to sort on&lt;br/&gt;
3) Fields used to perform range scan&lt;/p&gt;

&lt;p&gt;For more on this see the following blog post: &lt;a href=&quot;https://www.alexbevi.com/blog/2020/05/16/optimizing-mongodb-compound-indexes-the-equality-sort-range-esr-rule/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://www.alexbevi.com/blog/2020/05/16/optimizing-mongodb-compound-indexes-the-equality-sort-range-esr-rule/&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="4115844" author="david.storch" created="Mon, 11 Oct 2021 14:51:37 +0000"  >&lt;p&gt;I&apos;ll mark this for triage. We can consider as a team whether we want to resolve this ticket as a dupe of &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-13197&quot; title=&quot;Tighten index bounds and allow compound index to be chosen when predicate on leading field is not provided&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-13197&quot;&gt;SERVER-13197&lt;/a&gt;.&lt;/p&gt;</comment>
                            <comment id="743439" author="christopher.price@mtvn.com" created="Thu, 16 Oct 2014 19:08:16 +0000"  >&lt;p&gt;Use case:&lt;br/&gt;
I have a collection with 2.5 million documents.  The collection contains a directory of people.  Each person/document has a firstName, lastName, zipcode and some other ancillary fields.&lt;br/&gt;
I have a customer that wants to find the zipcodes of all people with the firstName of &quot;Adam&quot; or &quot;Eve&quot; and sort the results by lastName.&lt;br/&gt;
What is the optimal index(es) for the following queries?&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;db.directory.find({&quot;firstName&quot;:{$in:[&quot;Adam&quot;,&quot;Eve&quot;]}},{&quot;lastName&quot;:1,&quot;firstname&quot;:1,&quot;zipcode&quot;:1}).sort(&quot;lastName&quot;:1);&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;...&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;db.directory.find({&quot;firstName&quot;:{$in:[&quot;Adam&quot;,&quot;Eve&quot;]}}).count();&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;45678&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;In an odd twist, there are some people that don&apos;t have a lastName.&lt;/p&gt;</comment>
                            <comment id="740281" author="christopher.price@mtvn.com" created="Mon, 13 Oct 2014 23:47:42 +0000"  >&lt;p&gt;Actually, in the examples I provided that led to this ticket, the index on &lt;/p&gt;
{A:1, B:1}
&lt;p&gt; was correctly selected.  &lt;br/&gt;
The nscanned was the same as the count of the documents in the collection (possibly to be expected).&lt;br/&gt;
Additionally, nscannedObjects was also the same as the count of the documents in the collection which was not expected considering that MongoDB should have been able to retrieve and validate B from the index (that part of the query should be covered by the index).&lt;/p&gt;</comment>
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