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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-44487] Only pull looked-up fields into DocumentStorage</title>
                <link>https://jira.mongodb.org/browse/SERVER-44487</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;During query execution, when we need to lookup a &lt;tt&gt;Value&lt;/tt&gt; inside a &lt;tt&gt;Document&lt;/tt&gt;, there are two possible code paths:&lt;/p&gt;
&lt;ol&gt;
	&lt;li&gt;The &lt;tt&gt;Value&lt;/tt&gt; already resides in &lt;tt&gt;DocumentStorage&lt;/tt&gt;.&lt;/li&gt;
	&lt;li&gt;We need to create the &lt;tt&gt;Value&lt;/tt&gt; by reading the backing BSON object.&lt;/li&gt;
&lt;/ol&gt;


&lt;p&gt;The second case requires a linear scan of the BSON in order to find a &lt;tt&gt;BSONElement&lt;/tt&gt; with the correct field name. During this scan, we currently eagerly convert all of the &lt;tt&gt;BSONElements&lt;/tt&gt; we encounter into &lt;tt&gt;Values&lt;/tt&gt; which reside inside the &lt;tt&gt;DocumentSource&lt;/tt&gt; cache. This is done to ensure that only one forwards scan of the BSON object is necessary, avoiding potential quadratic performance required to look up &lt;em&gt;n&lt;/em&gt; fields in a &lt;tt&gt;Document&lt;/tt&gt;. However, our performance tests show that this eager conversion of &lt;tt&gt;BSONElement&lt;/tt&gt; to &lt;tt&gt;Value&lt;/tt&gt; is costly. In many cases, it is likely wasted work, since we might never need to read the field again later on. In order to improve performance, we should only pull the fields that the caller actually looked up into &lt;tt&gt;DocumentStorage&lt;/tt&gt;.&lt;/p&gt;

&lt;p&gt;In the future, if we find that repeated scans of the backing BSON are expensive, we can implement query analysis in order to compute a set of fields that are likely to be looked up later, and then pull these only these fields into the document&apos;s cache.&lt;/p&gt;</description>
                <environment></environment>
        <key id="993154">SERVER-44487</key>
            <summary>Only pull looked-up fields into DocumentStorage</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="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="david.storch@mongodb.com">David Storch</assignee>
                                    <reporter username="david.storch@mongodb.com">David Storch</reporter>
                        <labels>
                    </labels>
                <created>Thu, 7 Nov 2019 23:18:14 +0000</created>
                <updated>Sun, 29 Oct 2023 22:15:13 +0000</updated>
                            <resolved>Wed, 13 Nov 2019 17:31:21 +0000</resolved>
                                                    <fixVersion>4.3.2</fixVersion>
                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>1</watches>
                                                                                                                <comments>
                            <comment id="2538797" author="xgen-internal-githook" created="Wed, 13 Nov 2019 14:58:45 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;David Storch&apos;, &apos;username&apos;: &apos;dstorch&apos;, &apos;email&apos;: &apos;david.storch@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-44487&quot; title=&quot;Only pull looked-up fields into DocumentStorage&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-44487&quot;&gt;&lt;del&gt;SERVER-44487&lt;/del&gt;&lt;/a&gt; Only construct Values in DocumentStorage cache for looked-up fields.&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/5ae506f4fa83c27b74a38ee6c2f58fbaf680c25d&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/5ae506f4fa83c27b74a38ee6c2f58fbaf680c25d&lt;/a&gt;&lt;/p&gt;</comment>
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