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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-28312] take slice or element of array before materializing it </title>
                <link>https://jira.mongodb.org/browse/SERVER-28312</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;When a large array has format&lt;/p&gt;
&lt;p/&gt;
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&lt;p/&gt;

&lt;p&gt;and aggregation does something like&lt;/p&gt;
&lt;p/&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-top: 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;{$slice: [&quot;$a.b&quot;, 1, 2]}&lt;/span&gt;&lt;/pre&gt;
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&lt;p/&gt;

&lt;p&gt;then as array &quot;a&quot; gets larger, materializing it into &quot;$a.b&quot; becomes more expensive - and just taking $slice (or $arrayElemAt) and then extracting &quot;a.b&quot; can be significantly faster.&lt;/p&gt;

&lt;p&gt;Discovered when working with very large arrays (over 10,000) and doing $slice and two $arrayElemAt operations in the same stage and having it perform 100x worse than explicitly taking slice first and then extracting b out of smaller result.&lt;/p&gt;</description>
                <environment></environment>
        <key id="364785">SERVER-28312</key>
            <summary>take slice or element of array before materializing it </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="asya.kamsky@mongodb.com">Asya Kamsky</reporter>
                        <labels>
                            <label>optimization</label>
                            <label>performance</label>
                    </labels>
                <created>Tue, 14 Mar 2017 22:33:55 +0000</created>
                <updated>Tue, 6 Dec 2022 04:05:23 +0000</updated>
                                                                            <component>Aggregation Framework</component>
                                        <votes>0</votes>
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