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    <title>MongoDB Jira</title>
    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <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-68677] Skip row store projection in column index plans when possible</title>
                <link>https://jira.mongodb.org/browse/SERVER-68677</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;In the row store fallback path of column index plans, we apply a projection to the row store document to ensure that it has the same &quot;schema&quot; as the documents that would have been returned by the column index path. This is fairly expensive, and can often be skipped (including any time there is a $group above the column scan).&lt;/p&gt;

&lt;p&gt;This would build off of the analysis done in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-66061&quot; title=&quot;Optimize query plans using columnar indexes&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-66061&quot;&gt;&lt;del&gt;SERVER-66061&lt;/del&gt;&lt;/a&gt; where we detect cases where a projection is not necessary. In the same cases (unless I&apos;m forgetting an edge case), we should be able to skip adding this expression which projects the full document retrieved from the row store. That is added &lt;span class=&quot;error&quot;&gt;&amp;#91;here&amp;#93;&lt;/span&gt;(&lt;a href=&quot;https://github.com/10gen/mongo/blob/21b7dfd8f85224445da4f6098f22ac463ee18f72/src/mongo/db/query/sbe_stage_builder.cpp#L764&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/10gen/mongo/blob/21b7dfd8f85224445da4f6098f22ac463ee18f72/src/mongo/db/query/sbe_stage_builder.cpp#L764&lt;/a&gt;) unconditionally. We should be able to attach some boolean to the ColumnIndexScanNode which can say whether or not a projection is necessary. That same ColumnIndexScanNode should be available and in scope in &lt;span class=&quot;error&quot;&gt;&amp;#91;this analysis from SERVER-66061&amp;#93;&lt;/span&gt;(&lt;a href=&quot;https://github.com/10gen/mongo/blob/21b7dfd8f85224445da4f6098f22ac463ee18f72/src/mongo/db/query/planner_analysis.cpp#L424&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/10gen/mongo/blob/21b7dfd8f85224445da4f6098f22ac463ee18f72/src/mongo/db/query/planner_analysis.cpp#L424&lt;/a&gt;). There we could add a boolean flag to say &quot;I don&apos;t need an exact schema&quot;, in case there is a subsequent $group stage. &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=ian.boros%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;ian.boros@mongodb.com&quot;&gt;ian.boros@mongodb.com&lt;/a&gt; suggested calling this &quot;isStrict&quot; or something like that.&lt;/p&gt;

&lt;p&gt;We should be able to test this code path by inspecting the SBE plan in the explain output, where we should be able to tell if there is a row store expression or not. &lt;/p&gt;</description>
                <environment></environment>
        <key id="2109587">SERVER-68677</key>
            <summary>Skip row store projection in column index plans when possible</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="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="13201">Fixed</resolution>
                                        <assignee username="steve.tarzia@mongodb.com">Steve Tarzia</assignee>
                                    <reporter username="ian.boros@mongodb.com">Ian Boros</reporter>
                        <labels>
                    </labels>
                <created>Tue, 9 Aug 2022 14:30:48 +0000</created>
                <updated>Sun, 29 Oct 2023 21:34:44 +0000</updated>
                            <resolved>Fri, 23 Sep 2022 15:10:20 +0000</resolved>
                                                    <fixVersion>6.2.0-rc0</fixVersion>
                                                        <votes>0</votes>
                                    <watches>5</watches>
                                                                                                                <comments>
                            <comment id="4851195" author="xgen-internal-githook" created="Fri, 23 Sep 2022 15:07:43 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Steve Tarzia&apos;, &apos;email&apos;: &apos;steve.tarzia@mongodb.com&apos;, &apos;username&apos;: &apos;starzia&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-68677&quot; title=&quot;Skip row store projection in column index plans when possible&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-68677&quot;&gt;&lt;del&gt;SERVER-68677&lt;/del&gt;&lt;/a&gt; Skip row store projection in column scan plans when possible&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/538c5d15bf6f84fcfab9328fdf9857e120321e00&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/538c5d15bf6f84fcfab9328fdf9857e120321e00&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="4848195" author="JIRAUSER1263890" created="Thu, 22 Sep 2022 14:18:52 +0000"  >&lt;p&gt;I measured the performance impact of this change in the warm cache case and I saw a 14% speedup (143ms vs 163ms).&#160; I used the following query on the TPCH denormalized 1gb dataset:&lt;/p&gt;
&lt;p/&gt;
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&lt;p/&gt;
&lt;p&gt;Because $nation is an object, this will use the row store fallback every single time.&#160; I don&apos;t think there will be a measurable impact in the cold cache scenario.&lt;/p&gt;

&lt;p&gt;I was holding off merging my change until I saw a performance change.&#160; This looks significant enough to warrant the merge so I will merge it soon, after addressing the latest minor feedback in the code review.&lt;/p&gt;</comment>
                    </comments>
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                                        <issuelink>
            <issuekey id="2120534">SERVER-69105</issuekey>
        </issuelink>
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