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    <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-69125] SBE vs. classic engine selection rules differ between equivalent find and agg queries</title>
                <link>https://jira.mongodb.org/browse/SERVER-69125</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Consider these two queries:&lt;/p&gt;

&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;db.coll.aggregate([{$project: {a: [&quot;foo&quot;]}}]);&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;

&lt;p&gt;They are entirely identical in meaning and should return the same result set. The only difference between them is that the former is expressed as a find command and the latter is expressed as an aggregate command.&lt;/p&gt;

&lt;p&gt;Explaining both queries shows that the find command uses the classic execution engine and the aggregate command uses SBE. This difference could potentially surprise users, in particular by leading to different performance properties (or by leading to differences in diagnostic output, e.g. radically different explain output). Ideally we would guarantee that when a find is re-expressed as an aggregate or vice-versa, the classic vs. SBE engine selection rules do not also change.&lt;/p&gt;

&lt;p&gt;The reason for the different engine selection relates to whether SBE compatibility is checked before or after pipeline optimization. For find, the &lt;tt&gt;sbeCompatible&lt;/tt&gt; flag is set at parse time. For agg, the &lt;tt&gt;sbeCompatible&lt;/tt&gt; flag is set only after some optimizations have taken place. This happens because agg resets the &lt;tt&gt;sbeCompatible&lt;/tt&gt; flag to true &lt;a href=&quot;https://github.com/mongodb/mongo/blob/9ca61a6d178ec3bb2fe46c93e31c6ac3a503c75d/src/mongo/db/pipeline/pipeline_d.cpp#L270-L273&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;here&lt;/a&gt;. This is done to deal with the case where the full query has some SBE incompatible properties, but the portion of the query pushed down to the find layer is SBE compatible.&lt;/p&gt;

&lt;p&gt;In this particular example, [&quot;foo&quot;] is interpreted by the parser as an &lt;tt&gt;ExpressionArray&lt;/tt&gt;, which is SBE incompatible. However, it can subsequently be optimized to an &lt;tt&gt;ExpressionConstant&lt;/tt&gt; which effectively converts an SBE incompatible query into an SBE compatible one. This is the only case I&apos;m aware of where an optimization can render a formerly incompatible query compatible with SBE, but there could be others.&lt;/p&gt;</description>
                <environment></environment>
        <key id="2121578">SERVER-69125</key>
            <summary>SBE vs. classic engine selection rules differ between equivalent find and agg queries</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="david.storch@mongodb.com">David Storch</reporter>
                        <labels>
                    </labels>
                <created>Wed, 24 Aug 2022 20:41:18 +0000</created>
                <updated>Mon, 5 Dec 2022 23:48:32 +0000</updated>
                                                                                                <votes>0</votes>
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