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    <title>MongoDB Jira</title>
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    <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-79039] Plan cache entries can be evicted and replanned continuously for certain workload patterns</title>
                <link>https://jira.mongodb.org/browse/SERVER-79039</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;For certain patterns of queries, we can run into a cycle of cache-&amp;gt;evict-&amp;gt;replan-&amp;gt;cache for the same query shape and same plan depending on the parameters.  By default, the planner requires a 10x increase in works to trigger replanning, which manifests in the slow query logs as &lt;tt&gt;&quot;replanReason&quot;:&quot;cached plan was less efficient than expected: expected trial execution to take X reads but it took at least Y reads&quot;&lt;/tt&gt;.&lt;/p&gt;

&lt;p&gt;However, when the higher work plan triggers the eviction, the cache entry remains in an &quot;inactive&quot; state. If the next query to activate it has a lower works value, then we&apos;ll use the new one. This puts us in a state where its like the higher work query never happened, and its possible to evict and replan again. &lt;/p&gt;

&lt;p&gt;Note that the notion of an inactive cache entry was put in place for the opposite case: when a potentially sub-optimal, very high works plan gets cached. See &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; for more details on that.&lt;/p&gt;</description>
                <environment></environment>
        <key id="2394111">SERVER-79039</key>
            <summary>Plan cache entries can be evicted and replanned continuously for certain workload patterns</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="12300">Won&apos;t Do</resolution>
                                        <assignee username="christopher.harris@mongodb.com">Chris Harris</assignee>
                                    <reporter username="nicholas.zolnierz@mongodb.com">Nicholas Zolnierz</reporter>
                        <labels>
                    </labels>
                <created>Mon, 17 Jul 2023 21:19:45 +0000</created>
                <updated>Fri, 1 Sep 2023 19:33:36 +0000</updated>
                            <resolved>Thu, 17 Aug 2023 20:58:54 +0000</resolved>
                                                                                        <votes>0</votes>
                                    <watches>9</watches>
                                                                                                                <comments>
                            <comment id="5581803" author="conchi.bueno" created="Fri, 21 Jul 2023 01:34:30 +0000"  >&lt;p&gt;Hi &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=christopher.harris%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;christopher.harris@mongodb.com&quot;&gt;christopher.harris@mongodb.com&lt;/a&gt;&#160;&lt;/p&gt;

&lt;p&gt;I&apos;ve discussed this with &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=william.byrne%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;william.byrne@mongodb.com&quot;&gt;william.byrne@mongodb.com&lt;/a&gt;. We agree the changes to the replanning behaviour are too risky but in terms of diagnosability we could consider adding the original plan when a replanning happens.&lt;/p&gt;

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

&lt;p&gt;&#160;&lt;/p&gt;</comment>
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