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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-29059] Regex query with indexed $in clause use incorrect index</title>
                <link>https://jira.mongodb.org/browse/SERVER-29059</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;We appear to be hitting an issue where MongoDB is choosing to use a less efficient index when performing a regex query, that it actually could.&lt;/p&gt;

&lt;p&gt;Our data has the following structure&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;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;{&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;	title : &quot;String title&quot;,&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;	cms_title_sort : &quot;string title&quot;,&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;	tags : [&quot;foo&quot;, &quot;bar&quot;, &quot;baz&quot;],&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;	rank : 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;   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;
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			&lt;/tbody&gt;
&lt;/table&gt;
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&lt;p/&gt;
&lt;p&gt;With two indexes:&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;ensureIndex({ cms_title_sort : 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;   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;ensureIndex({ tags : 1, rank : 1, cms_title_sort : 1 })&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
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&lt;p/&gt;
&lt;p&gt;If we perform &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;find({ cms_title_sort : /keyword/, tags : { $in : [&quot;foo&quot;] } })&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;sometimes it uses the &lt;tt&gt;cms_title_sort&lt;/tt&gt; index, other times it uses the tags index. The table contains between 15k and 100k records, so not the largest of tables. When it uses the &lt;tt&gt;cms_title_sort&lt;/tt&gt; index, it&apos;s very fast and finishes in &amp;lt;100ms. When it uses the tags index, it can take up to a second. The reason for this is that it first uses the tags index (which have marginal cardinality), then it performs the regex component on the documents themselves in a FETCH step. When it uses the &lt;tt&gt;cms_title_sort&lt;/tt&gt; index, it performs the regex in memory, and then does the tag portion on the documents. Because the regex has a very high cardinality the result set is significantly smaller, and it goes quickly. In general I would think that most regex queries have high cardinality, so then, should that not instruct mongo to possible lean towards those indices?&lt;/p&gt;

&lt;p&gt;We have tried a few different index formats. According to the docs, it would seem that an index which looks like &lt;tt&gt;ensureIndex({ tags : 1, cms_title_sort : 1 })&lt;/tt&gt; should allow for a query to be entirely performed in memory. Even in that case, the system still performs the cms_title_sort component in a FETCH step, rather than in the index step where it checks the array. In the docs for indexed arrays, it mentions some other caveats, if they don&apos;t work with regex it may be best to call that as well.&lt;/p&gt;

&lt;p&gt;I have created a test script which shows the issue. Consistently with this test data the &lt;tt&gt;cms_title_sort&lt;/tt&gt; index performs the most optimally for all 3 queries, yet mongo sometimes chooses the tags index. In this example it&apos;s not as big of a problem, both queries are fairly fast, but as the data gets larger and more documents, then things can slow into the seconds. From my tests the index chosen changes depending on how many records you put in the db and the specific keys you attempt to search on.&lt;/p&gt;

&lt;p&gt;Lastly, we are using 3.2.6, this behavior may have changed in newer versions. If it is better in newer versions that would be great to know!&lt;/p&gt;</description>
                <environment></environment>
        <key id="380397">SERVER-29059</key>
            <summary>Regex query with indexed $in clause use incorrect index</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="13202">Works as Designed</resolution>
                                        <assignee username="mark.agarunov">Mark Agarunov</assignee>
                                    <reporter username="owenallenaz">Owen Allen</reporter>
                        <labels>
                    </labels>
                <created>Wed, 3 May 2017 22:49:34 +0000</created>
                <updated>Fri, 27 Oct 2023 13:54:28 +0000</updated>
                            <resolved>Fri, 19 May 2017 22:06:16 +0000</resolved>
                                    <version>3.2.6</version>
                                                    <component>Querying</component>
                                        <votes>1</votes>
                                    <watches>10</watches>
                                                                                                                <comments>
                            <comment id="1575873" author="mark.agarunov" created="Fri, 19 May 2017 21:49:43 +0000"  >&lt;p&gt;Hello &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=owenallenaz&quot; class=&quot;user-hover&quot; rel=&quot;owenallenaz&quot;&gt;owenallenaz&lt;/a&gt;,&lt;/p&gt;

&lt;p&gt;Thank you for the report. Looking over the information you&apos;ve provided, it seems that this is expected behavior. What you are describing may be due to the query planner caching the query plan.  The query planner will cache the plan for better performance, however if there is a large difference in the number of results returned between queries of similar shape, the query planner may be re run and choose a different index which would be more optimal for that query. This plan is then cached and may cause future queries to use this same index. &lt;/p&gt;

&lt;p&gt;To avoid this, &lt;a href=&quot;https://docs.mongodb.com/manual/reference/command/planCacheSetFilter/#dbcmd.planCacheSetFilter&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;you may be able to use &lt;tt&gt;planCacheSetFilter&lt;/tt&gt;&lt;/a&gt; to have more deterministic query execution.&lt;/p&gt;

&lt;p&gt;Thanks,&lt;br/&gt;
Mark&lt;/p&gt;</comment>
                    </comments>
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                            <attachment id="155300" name="mongoIndexTest.js" size="1788" author="owenallenaz" created="Wed, 3 May 2017 22:48:01 +0000"/>
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