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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-12222] Hashed index is not used for queries that can be rewritten as equalities</title>
                <link>https://jira.mongodb.org/browse/SERVER-12222</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Create a hashed index on field &apos;a&apos;:&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;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.t.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;   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.t.ensureIndex({a: 1})&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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&lt;p/&gt;
&lt;p&gt;Now consider the following 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;db.t.find({a: {$gte: 3, $lte: 3}})&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
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&lt;p/&gt;
&lt;p&gt;The old query system recognized that this query means &quot;a == 3&quot; and therefore was able to take advantage of the hashed index. In the new query system, however, this query will just do a collection scan instead of generating an indexed solution. A jstest for this (&lt;a href=&quot;https://github.com/mongodb/mongo/blob/6486c53c2de908d90f098eb1295a07440a8cc6b0/jstests/hashindex1.js#L43&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;see here&lt;/a&gt;) was disabled as part of migrating to the new query system.&lt;/p&gt;

&lt;p&gt;The problem with the new system occurs inside PlannerIXSelect::rateIndices. Here, the planner checks whether the hashed index is &quot;compatible&quot; with the subexpressions &quot;a &amp;gt;= 3&quot; and &quot;a &amp;lt;= 3&quot;. But compatibility for hashed indices is defined as being an equality match expression or an $in (&lt;a href=&quot;https://github.com/mongodb/mongo/blob/6486c53c2de908d90f098eb1295a07440a8cc6b0/src/mongo/db/query/planner_ixselect.cpp#L121&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;see the code here&lt;/a&gt;).&lt;/p&gt;

&lt;p&gt;This is a non-trivial problem to fix because the IX selection stage of the query planner happens before the index bounds builder runs. It is the index bounds builder that would normally take the intersection of the &quot;a &amp;gt;= 3&quot; and &quot;a &amp;lt;= 3&quot;, thus recognizing that the query can be executed as an index scan over a point interval.&lt;/p&gt;</description>
                <environment></environment>
        <key id="104026">SERVER-12222</key>
            <summary>Hashed index is not used for queries that can be rewritten as equalities</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</type>
                                            <priority id="4" iconUrl="https://jira.mongodb.org/images/icons/priorities/minor.svg">Minor - P4</priority>
                        <status id="10038" iconUrl="https://jira.mongodb.org/images/icons/subtask.gif" description="">Backlog</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-query-optimization">Backlog - Query Optimization</assignee>
                                    <reporter username="david.storch@mongodb.com">David Storch</reporter>
                        <labels>
                    </labels>
                <created>Tue, 31 Dec 2013 18:49:02 +0000</created>
                <updated>Tue, 6 Dec 2022 05:13:43 +0000</updated>
                                            <version>2.5.4</version>
                                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>2</watches>
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