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        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-81868] SBE $group implementation still scales poorly with number of accumulators</title>
                <link>https://jira.mongodb.org/browse/SERVER-81868</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;If we run a simple test with a $group by query with many accumulators, SBE performs worse than classic, and the gap appears to increase as the number of accumulators grows.&lt;/p&gt;

&lt;p&gt;For many queries the runtime is dominated by other work besides the accumulators (reading data, evaluating other expressions, etc). In these cases, the &quot;regression&quot; in time spent accumulating may not be visible at all. On the other hand, when running the accumulators is a large fraction of the query runtime, there is a clear difference.&lt;/p&gt;

&lt;p&gt;Currently the only way to see this issue is through queries with a large (20+) number of a accumulators. However, when running a time series $group query in SBE, we see similar behavior. This is because with time series, the amount of work done to read each document is relatively small, so the $group-by processing represents a greater fraction of the runtime.&lt;/p&gt;

&lt;p&gt;The issue appears to be most severe with $avg, presumably because the SBE implementation decomposes this into two separate accumulators (sum and count).&lt;/p&gt;
</description>
                <environment></environment>
        <key id="2459427">SERVER-81868</key>
            <summary>SBE $group implementation still scales poorly with number of accumulators</summary>
                <type id="3" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14718&amp;avatarType=issuetype">Task</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="12452" iconUrl="https://jira.mongodb.org/images/icons/statuses/generic.png" description="">Needs Scheduling</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-query-execution">Backlog - Query Execution</assignee>
                                    <reporter username="ian.boros@mongodb.com">Ian Boros</reporter>
                        <labels>
                    </labels>
                <created>Wed, 4 Oct 2023 18:07:39 +0000</created>
                <updated>Thu, 5 Oct 2023 01:00:42 +0000</updated>
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