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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-43657] Investigate whether there are situations in which we don&apos;t need to set batchSize to 0 when opening a cursor for a pipeline</title>
                <link>https://jira.mongodb.org/browse/SERVER-43657</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;It looks like we do know whether an aggregate needs to be merged on a shard (&lt;a href=&quot;https://github.com/mongodb/mongo/blob/master/src/mongo/db/pipeline/sharded_agg_helpers.cpp#L222&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;here&lt;/a&gt;) before we set the batchSize (&lt;a href=&quot;https://github.com/mongodb/mongo/blob/master/src/mongo/db/pipeline/sharded_agg_helpers.cpp#L271&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;in this function&lt;/a&gt;, &lt;a href=&quot;https://github.com/mongodb/mongo/blob/master/src/mongo/db/pipeline/sharded_agg_helpers.cpp#L190&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;here&lt;/a&gt;). If we pass that value through to the function, we may be able to eliminate the need for a request to be sent with batchSize: 0&lt;/p&gt;</description>
                <environment></environment>
        <key id="942158">SERVER-43657</key>
            <summary>Investigate whether there are situations in which we don&apos;t need to set batchSize to 0 when opening a cursor for a pipeline</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="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-optimization">Backlog - Query Optimization</assignee>
                                    <reporter username="ted.tuckman@mongodb.com">Ted Tuckman</reporter>
                        <labels>
                    </labels>
                <created>Thu, 26 Sep 2019 17:54:21 +0000</created>
                <updated>Wed, 4 Oct 2023 14:54:34 +0000</updated>
                                                                            <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>21</watches>
                                                                                                                <comments>
                            <comment id="2443717" author="charlie.swanson" created="Tue, 1 Oct 2019 19:25:30 +0000"  >&lt;p&gt;One thing to consider here: If we use a non-zero batchSize for establishing the cursors on the shards, then there&apos;s a possibility it will interact poorly with the shard versioning protocol and end up wasting more work. There is a benefit to doing as little as possible before verifying that all shards are on the same page with respect to shard versioning. &lt;/p&gt;

&lt;p&gt;For example, if you had 10 shards and some active migrations, 8 shards might complete and get the first batch but 2 shards may have recently completed a migration so respond with an error. This means the whole aggregation needs to be retried, throwing out any work that those other 8 shards have done. We also have no way of interrupting the work on those other shards right now that I know of. We don&apos;t have a cursor ID yet, and no op ID that we can guess. So in the worst case these 8 shards will compute an hours-long grouping query to return the first batch only to then realize no one is interested in that response anymore.&lt;/p&gt;

&lt;p&gt;That all said, for simple streaming queries this is likely a pretty good idea, especially if there&apos;s a limit. My understanding is that this is something we do for normal queries (find commands), just not for aggregates.&lt;/p&gt;</comment>
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            <issuekey id="660448">SERVER-38761</issuekey>
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            <issuekey id="1111934">SERVER-45738</issuekey>
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