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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-32452] Replanning may not occur when a plan with an extremely high &apos;works&apos; value is cached</title>
                <link>https://jira.mongodb.org/browse/SERVER-32452</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;div class=&quot;panel&quot; style=&quot;background-color: #eeeeee;border-color: #cccccc;border-width: 1px;&quot;&gt;&lt;div class=&quot;panelHeader&quot; style=&quot;border-bottom-width: 1px;border-bottom-color: #cccccc;background-color: #6cb33f;&quot;&gt;&lt;b&gt;Issue Status as of Aug 28, 2018&lt;/b&gt;&lt;/div&gt;&lt;div class=&quot;panelContent&quot; style=&quot;background-color: #eeeeee;&quot;&gt;
&lt;p&gt;&lt;b&gt;ISSUE DESCRIPTION AND IMPACT&lt;/b&gt;&lt;br/&gt;
Replanning may not work when a slow cached plan with a high &lt;tt&gt;works&lt;/tt&gt; value is inferior to a potential plan.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;DIAGNOSIS AND AFFECTED VERSIONS&lt;/b&gt;&lt;br/&gt;
This can be identified by viewing the &lt;tt&gt;mongod&lt;/tt&gt; logs for slow queries that utilize an IXSCAN, with an excessively high number of keysExamined, and comparing the number of keysExamined against the number in the output of the &lt;tt&gt;explain(true)&lt;/tt&gt; command. &lt;tt&gt;explain(true)&lt;/tt&gt; output for the exact same query (with the same predicate values) will have a much smaller number of keysExamined, or may use a different index entirely. &lt;/p&gt;

&lt;p&gt;This issue affects MongoDB versions 3.2, 3.4, 3.6 and 4.0.&lt;/p&gt;

&lt;p&gt;&lt;b&gt;REMEDIATIONS AND WORKAROUNDS&lt;/b&gt;&lt;br/&gt;
Users affected by suboptimal query plan selection may:&lt;/p&gt;

&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;Specify explicit &lt;a href=&quot;http://docs.mongodb.org/manual/reference/method/cursor.hint/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;hints&lt;/a&gt; for queries affected by this issue.&lt;/li&gt;
	&lt;li&gt;Set a &lt;a href=&quot;http://docs.mongodb.org/manual/reference/command/planCacheSetFilter/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;query plan cache index filter&lt;/a&gt; to override default query planning.&lt;/li&gt;
	&lt;li&gt;Review current &lt;a href=&quot;https://docs.mongodb.com/manual/applications/indexes/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;indexing strategies&lt;/a&gt; for potential improvements, such as &lt;a href=&quot;https://docs.mongodb.com/manual/tutorial/sort-results-with-indexes/#sort-and-non-prefix-subset-of-an-index&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;filtering and sorting with a single index&lt;/a&gt;.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;&lt;b&gt;RESOLUTION DETAILS&lt;/b&gt;&lt;br/&gt;
The fix for this issue introduces the notion of &quot;inactive&quot; and &quot;active&quot; cache entries. Cache entries are first created in an &quot;inactive&quot; state, and are not used by the planner. They are only used to keep track of the expected number of &lt;tt&gt;works&lt;/tt&gt; that the query will take.&lt;/p&gt;

&lt;p&gt;When a plan is run, it is evaluated against the existing inactive entry:&lt;/p&gt;

&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;If the plan&apos;s &lt;tt&gt;works&lt;/tt&gt; value is less than or equal to the inactive cache entry&apos;s &lt;tt&gt;works&lt;/tt&gt; value, the new plan is put in an &quot;active&quot; entry, and the &lt;tt&gt;works&lt;/tt&gt; value is updated.&lt;/li&gt;
	&lt;li&gt;If the new plan&apos;s &lt;tt&gt;works&lt;/tt&gt; value is greater than the inactive entry&apos;s value, the new plan is not cached. Instead, the &lt;tt&gt;works&lt;/tt&gt; value of the inactive entry is multiplied by two (by default). The multiplier can be adjusted via the &lt;tt&gt;InternalQueryCacheWorksGrowthCoefficient&lt;/tt&gt; server parameter.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;&lt;b&gt;FIX VERSIONS&lt;/b&gt;&lt;br/&gt;
This issue is fixed in MongoDB 4.1.1, and will be available in the MongoDB 4.2 production release.&lt;/p&gt;
&lt;/div&gt;&lt;/div&gt;

&lt;h6&gt;&lt;a name=&quot;Originaldescription&quot;&gt;&lt;/a&gt;Original description&lt;/h6&gt;
&lt;p&gt;This will help give more control over query plans that get selected suboptimally.&lt;/p&gt;

&lt;p&gt;Could be a tunable parameter for the number of works.&lt;/p&gt;</description>
                <environment></environment>
        <key id="475498">SERVER-32452</key>
            <summary>Replanning may not occur when a plan with an extremely high &apos;works&apos; value is cached</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="13201">Fixed</resolution>
                                        <assignee username="ian.boros@mongodb.com">Ian Boros</assignee>
                                    <reporter username="andre.defrere@mongodb.com">Andre de Frere</reporter>
                        <labels>
                    </labels>
                <created>Fri, 22 Dec 2017 02:51:26 +0000</created>
                <updated>Mon, 30 Oct 2023 23:09:43 +0000</updated>
                            <resolved>Thu, 14 Jun 2018 18:38:40 +0000</resolved>
                                    <version>3.6.0</version>
                                    <fixVersion>4.1.1</fixVersion>
                                    <component>Querying</component>
                                        <votes>2</votes>
                                    <watches>24</watches>
                                                                                                                <comments>
                            <comment id="2031645" author="david.storch" created="Fri, 12 Oct 2018 17:15:15 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=jose.morales&quot; class=&quot;user-hover&quot; rel=&quot;jose.morales&quot;&gt;jose.morales&lt;/a&gt;,&lt;/p&gt;

&lt;p&gt;Unfortunately, due to the complexity of this change, it cannot be backported to 4.0, 3.6, or earlier versions. Users who wish to take advantage of this fix should upgrade to stable version 4.2 when it becomes available. Thanks for your interest in MongoDB, and let me know if you have any further questions!&lt;/p&gt;

&lt;p&gt;Best,&lt;br/&gt;
Dave&lt;/p&gt;
</comment>
                            <comment id="2031130" author="jose.morales" created="Fri, 12 Oct 2018 12:15:12 +0000"  >&lt;p&gt;Is in the roadmap to backport this fix to mongo 3.6.x ?&lt;/p&gt;</comment>
                            <comment id="1920703" author="xgen-internal-githook" created="Thu, 14 Jun 2018 15:25:05 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Ian Boros&apos;, &apos;email&apos;: &apos;ian.boros@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-32452&quot; title=&quot;Replanning may not occur when a plan with an extremely high &amp;#39;works&amp;#39; value is cached&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-32452&quot;&gt;&lt;del&gt;SERVER-32452&lt;/del&gt;&lt;/a&gt; add ability for plan cache entries to be inactive&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/2a66542157d5eac9ad43c749f3531bd57097cd26&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/2a66542157d5eac9ad43c749f3531bd57097cd26&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1881700" author="david.storch" created="Thu, 3 May 2018 15:14:08 +0000"  >&lt;p&gt;&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;, let&apos;s use this ticket to implement the &quot;tombstone&quot; policy described by the design for this project. This new policy should be flag-gated behind &lt;tt&gt;internalQueryCacheDisableTombstones&lt;/tt&gt; and off by default in 4.0. This ticket tracks correctness tests for the tombstone policy. Performance tests will be implemented as a separate work item.&lt;/p&gt;

&lt;p&gt;CC &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=charlie.swanson&quot; class=&quot;user-hover&quot; rel=&quot;charlie.swanson&quot;&gt;charlie.swanson&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1787916" author="christopher.harris" created="Mon, 29 Jan 2018 17:29:36 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=david.storch&quot; class=&quot;user-hover&quot; rel=&quot;david.storch&quot;&gt;david.storch&lt;/a&gt;, indeed this ticket is related to plans getting &quot;stuck&quot; in the cache.  After additional consideration this seems closely related to &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-19835&quot; title=&quot;SubplanStage should not cache plan for an individual $or branch if plans tie or there are no query results&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-19835&quot;&gt;&lt;del&gt;SERVER-19835&lt;/del&gt;&lt;/a&gt; (&quot;&lt;em&gt;SubplanStage should not cache plan for an individual $or branch if plans tie or there are no query results&lt;/em&gt;&quot;).  That ticket description notes:&lt;/p&gt;

&lt;blockquote&gt;
&lt;p&gt;2. the winning plan does not produce any results during the MultiPlanStage trial period.&lt;/p&gt;

&lt;p&gt;The intention behind this behavior is that in these edge cases the system has low confidence that it has selected the best plan. In order to avoid re-using the wrong plan from the cache for a subsequent execution of the query shape, the system refrains from creating a potentially erroneous cache entry.&lt;/p&gt;

&lt;p&gt;In version 3.1.2 under &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-15225&quot; title=&quot;CachedPlanStage should execute for trial period and re-plan if query performs poorly&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-15225&quot;&gt;&lt;del&gt;SERVER-15225&lt;/del&gt;&lt;/a&gt;, behavior was introduced to aggressively evict bad cache entries. This aggressive eviction obviated the need for not caching in case of ties or zero results, and therefore the system changed to &lt;em&gt;always&lt;/em&gt; create a cache entry.&lt;/p&gt;&lt;/blockquote&gt;

&lt;p&gt;Based on further thought and testing, I can&apos;t convince myself that the statement that the &quot;&lt;em&gt;aggressive eviction obviated the need for not caching in case of ... zero results&lt;/em&gt;&quot; is true.&lt;/p&gt;

&lt;p&gt;Consider this in conjunction with &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-31078&quot; title=&quot;Query planning is very slow during multiplanning when result set is empty&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-31078&quot;&gt;SERVER-31078&lt;/a&gt; (&quot;&lt;em&gt;Query planning is very slow during multiplanning when result set is empty&lt;/em&gt;&quot;).  The summary there is:&lt;/p&gt;

&lt;blockquote&gt;
&lt;p&gt;The &lt;tt&gt;MultiPlanStage&lt;/tt&gt; picks the best plan by &lt;a href=&quot;https://github.com/mongodb/mongo/blob/e7837911c89af144fe012e5063f8ca88c4c66956/src/mongo/db/exec/multi_plan.cpp#L211-L212&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&quot;working all the plans, stopping when a plan hits EOF or returns some fixed number of results&quot;&lt;/a&gt;. But if the query returns no results, this stopping condition can take a very long time and we&apos;ve essentially done a full index scan before returning.&lt;/p&gt;&lt;/blockquote&gt;

&lt;p&gt;My understanding of re-planning (referred to as &quot;aggressive cache eviction&quot; above I believe) is that the threshold will be set at 10x the number of works required when the plan was cached per the &lt;a href=&quot;https://github.com/mongodb/mongo/blob/r3.2.4/src/mongo/db/query/query_knobs.cpp#L45&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;internalQueryCacheEvictionRatio&lt;/tt&gt; parameter&lt;/a&gt;.  In the case where we do a full, or at least significant, scan of the index, the works threshold will be sufficiently high that it can never be reached.  &lt;/p&gt;

&lt;p&gt;This can happen, for example, if there is a predicate that is not one of the index keys and doesn&apos;t match any results.  Consider the following schema/indexing:&lt;/p&gt;

&lt;p/&gt;
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					&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;		&quot;name&quot; : &quot;_id_&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;		&quot;ns&quot; : &quot;SERVER-32452.demo&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;	},&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;		&quot;v&quot; : 2,&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;		&quot;key&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;			&quot;location&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;			&quot;sqft&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;		&quot;name&quot; : &quot;location_1_sqft_1&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;		&quot;ns&quot; : &quot;SERVER-32452.demo&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;	},&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;		&quot;v&quot; : 2,&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;		&quot;key&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;			&quot;location&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;			&quot;price&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;		&quot;name&quot; : &quot;location_1_price_1&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;		&quot;ns&quot; : &quot;SERVER-32452.demo&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;	}&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 let&apos;s say that the query shape in question is:&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;{location:&apos;NYC&apos;, sqft: {$gte:0, $lte:10000}, price: {$gte:0, $lte:120000}, baths: {$gte:0, $lte:10}}&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;Going back and forth between a user who is selective about square footage an a user that is selective about price allows the database to re-plan appropriately:&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;2018-01-29T10:48:04.549-0600 I COMMAND  [conn4] command SERVER-32452.demo appName: &quot;MongoDB Shell&quot; command: find { find: &quot;demo&quot;, filter: { location: &quot;NYC&quot;, sqft: { $gte: 0.0, $lte: 10000.0 }, price: { $gte: 0.0, $lte: 120000.0 }, baths: { $gte: 0.0, $lte: 10.0 } } } planSummary: IXSCAN { location: 1, price: 1 } cursorid:64815161403 keysExamined:101 docsExamined:101 fromMultiPlanner:1 numYields:2 nreturned:101 reslen:8969 locks:{ Global: { acquireCount: { r: 6 } }, Database: { acquireCount: { r: 3 } }, Collection: { acquireCount: { r: 3 } } } protocol:op_command 5ms&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;2018-01-29T10:48:16.249-0600 I COMMAND  [conn4] command SERVER-32452.demo appName: &quot;MongoDB Shell&quot; command: find { find: &quot;demo&quot;, filter: { location: &quot;NYC&quot;, sqft: { $gte: 9000.0, $lte: 10000.0 }, price: { $gte: 0.0, $lte: 1000000.0 }, baths: { $gte: 0.0, $lte: 10.0 } } } planSummary: IXSCAN { location: 1, sqft: 1 } cursorid:65262064703 keysExamined:101 docsExamined:101 fromMultiPlanner:1 replanned:1 numYields:12 nreturned:101 reslen:8969 locks:{ Global: { acquireCount: { r: 26 } }, Database: { acquireCount: { r: 13 } }, Collection: { acquireCount: { r: 13 } } } protocol:op_command 39ms&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;2018-01-29T10:48:27.209-0600 I COMMAND  [conn4] command SERVER-32452.demo appName: &quot;MongoDB Shell&quot; command: find { find: &quot;demo&quot;, filter: { location: &quot;NYC&quot;, sqft: { $gte: 0.0, $lte: 10000.0 }, price: { $gte: 0.0, $lte: 120000.0 }, baths: { $gte: 0.0, $lte: 10.0 } } } planSummary: IXSCAN { location: 1, price: 1 } cursorid:65543908337 keysExamined:101 docsExamined:101 fromMultiPlanner:1 replanned:1 numYields:10 nreturned:101 reslen:8969 locks:{ Global: { acquireCount: { r: 22 } }, Database: { acquireCount: { r: 11 } }, Collection: { acquireCount: { r: 11 } } } protocol:op_command 7ms&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;Now let&apos;s say someone comes along and would like a large place with a lot of bathrooms.  That filter on the non-indexed field is going to result in examining (and rejecting) plenty of documents.  This will trigger re-planning and then end up caching the new work metrics.  &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;2018-01-29T11:10:25.320-0600 I COMMAND  [conn4] command SERVER-32452.demo appName: &quot;MongoDB Shell&quot; command: find { find: &quot;demo&quot;, filter: { location: &quot;NYC&quot;, sqft: { $gte: 8000.0, $lte: 10000.0 }, price: { $gte: 0.0, $lte: 1000000.0 }, baths: { $gte: 8.0, $lte: 10.0 } } } planSummary: IXSCAN { location: 1, sqft: 1 } keysExamined:3022 docsExamined:3022 fromMultiPlanner:1 replanned:1 cursorExhausted:1 numYields:55 nreturned:0 reslen:90 locks:{ Global: { acquireCount: { r: 112 } }, Database: { acquireCount: { r: 56 } }, Collection: { acquireCount: { r: 56 } } } protocol:op_command 20ms&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;Now our re-planning threshold is set to ~30k documents, so our previous query predicates (smaller range on &lt;tt&gt;price&lt;/tt&gt;) are now doing lots of extra work:  &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;2018-01-29T11:11:53.234-0600 I COMMAND  [conn4] command SERVER-32452.demo appName: &quot;MongoDB Shell&quot; command: find { find: &quot;demo&quot;, filter: { location: &quot;NYC&quot;, sqft: { $gte: 0.0, $lte: 10000.0 }, price: { $gte: 0.0, $lte: 120000.0 }, baths: { $gte: 0.0, $lte: 10.0 } } } planSummary: IXSCAN { location: 1, sqft: 1 } cursorid:67256109411 keysExamined:4460 docsExamined:4460 numYields:35 nreturned:101 reslen:8969 locks:{ Global: { acquireCount: { r: 72 } }, Database: { acquireCount: { r: 36 } }, Collection: { acquireCount: { r: 36 } } } protocol:op_command 12ms&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;Depending on the data, it is still possible to trigger the re-planning mechanism at this point, but the &lt;tt&gt;price&lt;/tt&gt; predicate has to be pretty narrow.  Eg something along the lines of:&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;2018-01-29T11:20:39.902-0600 I COMMAND  [conn4] command SERVER-32452.demo appName: &quot;MongoDB Shell&quot; command: find { find: &quot;demo&quot;, filter: { location: &quot;NYC&quot;, sqft: { $gte: 0.0, $lte: 10000.0 }, price: { $gte: 0.0, $lte: 101000.0 }, baths: { $gte: 0.0, $lte: 10.0 } } } planSummary: IXSCAN { location: 1, price: 1 } cursorid:68018479453 keysExamined:101 docsExamined:101 fromMultiPlanner:1 replanned:1 numYields:239 nreturned:101 reslen:8969 locks:{ Global: { acquireCount: { r: 480 } }, Database: { acquireCount: { r: 240 } }, Collection: { acquireCount: { r: 240 } } } protocol:op_command 44ms&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, if a user were to come in and ONLY care about the non-indexed field (&lt;tt&gt;baths&lt;/tt&gt; in this case), then we can do a large scan that would set the threshold much higher than could ever be achieved.  Indeed, if this were a full index scan (such as could happen with &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-31078&quot; title=&quot;Query planning is very slow during multiplanning when result set is empty&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-31078&quot;&gt;SERVER-31078&lt;/a&gt;) then the threshold would be 10x the number of documents in the collection.&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;2018-01-29T11:21:46.417-0600 I COMMAND  [conn4] command SERVER-32452.demo appName: &quot;MongoDB Shell&quot; command: find { find: &quot;demo&quot;, filter: { location: &quot;NYC&quot;, sqft: { $gte: 0.0, $lte: 10000.0 }, price: { $gte: 0.0, $lte: 1000000.0 }, baths: { $gte: 8.0, $lte: 10.0 } } } planSummary: IXSCAN { location: 1, price: 1 } keysExamined:100001 docsExamined:100001 fromMultiPlanner:1 replanned:1 cursorExhausted:1 numYields:1033 nreturned:0 reslen:90 locks:{ Global: { acquireCount: { r: 2068 } }, Database: { acquireCount: { r: 1034 } }, Collection: { acquireCount: { r: 1034 } } } protocol:op_command 262ms&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;I&apos;ve also linked an interesting variation of a potential cause related to background index builds in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-32943&quot; title=&quot;Query during background index build can lead to suboptimal cached plan&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-32943&quot;&gt;&lt;del&gt;SERVER-32943&lt;/del&gt;&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;Hopefully this helps, but please let me know if I can provide further clarification.&lt;/p&gt;</comment>
                            <comment id="1760855" author="david.storch" created="Thu, 28 Dec 2017 16:58:14 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=andre.defrere&quot; class=&quot;user-hover&quot; rel=&quot;andre.defrere&quot;&gt;andre.defrere&lt;/a&gt;, is this suggested change motivated by some observed bad behavior? I&apos;m disinclined to mess with our plan selection logic unless we have empirical evidence that it improves plan selection, without causing plan selection to regress for certain workloads.&lt;/p&gt;

&lt;p&gt;Do I infer correctly that this is a suggested fix for bad plans getting stuck in the plan cache?&lt;/p&gt;</comment>
                    </comments>
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