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        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-55417] Conditionally reorder $sort and $lookup depending on whether the query planner allows for an index-provided sort</title>
                <link>https://jira.mongodb.org/browse/SERVER-55417</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;With the &lt;a href=&quot;https://github.com/mongodb/mongo/blob/ed8f7878acb8ac25b453499dee2d8e42166a4522/jstests/noPassthroughWithMongod/lookup_with_limit.js#L72&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;pipeline&lt;/a&gt; of &lt;span class=&quot;error&quot;&gt;&amp;#91;$lookup, $unwind, $sort, $limit&amp;#93;&lt;/span&gt;, the current &lt;a href=&quot;https://github.com/mongodb/mongo/blob/ed8f7878acb8ac25b453499dee2d8e42166a4522/src/mongo/db/pipeline/document_source_lookup.cpp#L477&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;implementation&lt;/a&gt; will not push $sort before $lookup since $unwind will block $limit from being pushed before $lookup and then combined with $sort. Although letting $sort take advantages of the indexes benefits more than absorbing $limit into $sort to run a top-k sort, there is no way in the current system to learn if there are indexes on the sorting fields.&lt;/p&gt;

&lt;p&gt;Instead of depending on the presence of $unwind, the optimization should reorder $lookup and $sort depending on if an index-provided sort is allowed. The other condition of not sorting on fields introduced or modified by $lookup and $unwind remains the same.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1655422">SERVER-55417</key>
            <summary>Conditionally reorder $sort and $lookup depending on whether the query planner allows for an index-provided sort</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="yuhong.zhang@mongodb.com">Yuhong Zhang</reporter>
                        <labels>
                            <label>optimization</label>
                    </labels>
                <created>Mon, 22 Mar 2021 16:28:46 +0000</created>
                <updated>Tue, 6 Dec 2022 01:29:55 +0000</updated>
                                                                                                <votes>0</votes>
                                    <watches>5</watches>
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                                                                <inwardlinks description="is related to">
                                        <issuelink>
            <issuekey id="320662">SERVER-26442</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="1634798">SERVER-54822</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="1634805">SERVER-54823</issuekey>
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