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        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-43944] Accumulators (and possibly expressions) can allocate large amounts of memory</title>
                <link>https://jira.mongodb.org/browse/SERVER-43944</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Generally every stage which builds some large data structure (think $sort or $group) will use estimated sizes to cap that memory usage to 100MB. Nothing like this exists for expressions or accumulators, but something like a $push can end up allocating lots of memory.&lt;/p&gt;

&lt;p&gt;For example, the pipeline:&lt;/p&gt;
&lt;p/&gt;
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&lt;p/&gt;
&lt;p&gt;will push everything to one giant array. This actually will error if spilling to disk is not enabled, since $group will approximate the size of all accumulators. But once we spill to disk and then begin merging back together there is no memory usage tracking. Because $push tracks just a &lt;tt&gt;vector&amp;lt;Value&amp;gt;&lt;/tt&gt;, that array itself isn&apos;t going to grow very large (each Value is just 16 bytes). However each of those Values indicates a sub-tree of memory that can add up to lots and lots of memory in the worst case.&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-44174&quot; title=&quot;$push and $addToSet should restrict memory usage&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-44174&quot;&gt;&lt;del&gt;SERVER-44174&lt;/del&gt;&lt;/a&gt; tracks the work to fix $push and $addToSet, while this ticket tracks the work to create a more general solution for all expressions and accumulators.&lt;/p&gt;</description>
                <environment></environment>
        <key id="966756">SERVER-43944</key>
            <summary>Accumulators (and possibly expressions) can allocate large amounts of memory</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="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-execution">Backlog - Query Execution</assignee>
                                    <reporter username="charlie.swanson@mongodb.com">Charlie Swanson</reporter>
                        <labels>
                    </labels>
                <created>Thu, 10 Oct 2019 17:57:12 +0000</created>
                <updated>Mon, 28 Aug 2023 17:10:26 +0000</updated>
                                                                            <component>Aggregation Framework</component>
                                        <votes>0</votes>
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