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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-83474] Can we avoid double-parsing a positional projection? Or alternatively can we afford to always double-parse the projection?</title>
                <link>https://jira.mongodb.org/browse/SERVER-83474</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;From a comment:&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;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&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;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: #008200; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;// Sadly, in some cases the match details cannot be generated from the unoptimized&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&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;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: #008200; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;// MatchExpression. For example, a rooted-$or of equalities won&apos;t work to produce the&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&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;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: #008200; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;// details, but if you optimize that query to an $in, it will work. If we were starting&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&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;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: #008200; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;// from scratch, we may disallow this. But it has already been released as working so we&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&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;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: #008200; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;// will keep it so, and here have to re-parse the projection using the new, normalized&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&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: #008200; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;// MatchExpression, before we save this projection for later execution.&lt;/span&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;&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;
&lt;p&gt;During code review, we thought:&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=charlie.swanson%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;charlie.swanson@mongodb.com&quot;&gt;charlie.swanson@mongodb.com&lt;/a&gt;: This is troublesome. I welcome other suggestions but this was the best I could think of. Always re-parsing seems like it would defeat the purpose. Another option I considered was to just use the optimized MatchExpression for the query shape for positional projections, but that&apos;s a strange edge case that would then become user-visible&lt;/p&gt;&lt;/blockquote&gt;
&lt;blockquote&gt;&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=davis.haupt%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;davis.haupt@mongodb.com&quot;&gt;davis.haupt@mongodb.com&lt;/a&gt;: I&apos;d be curious to see if we can measure the perf impact of this. My first guess would be that parsing projections isn&apos;t that expensive relative to filters, but that intuition may be wrong.&lt;/p&gt;&lt;/blockquote&gt;
&lt;blockquote&gt;&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=jacob.evans%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;jacob.evans@mongodb.com&quot;&gt;jacob.evans@mongodb.com&lt;/a&gt;: what are the details? the bit that interacts with positional projection? I&apos;m tempted to leave this and file a ticket to make it so that requiresMatchDetails() can&apos;t silently fail with certain inputs, we should probably check with the stakeholders (thinking Andy) to make sure it&apos;s okay to make this type of query slower, I think it&apos;s worth asking since this doesn&apos;t seem like our bread and butter... and we might want to add a workload if one doesn&apos;t exist...&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;For this ticket, we should see the performance impact in two cases:&lt;/p&gt;
&lt;ol&gt;
	&lt;li&gt;If we always double-parse can we measure anything actually getting slower?&lt;/li&gt;
	&lt;li&gt;Do we have any benchmarks with positional projections? Can we test the impact on avoiding this double-parse there? Do we need to create new benchmarks to test it? For this testing, it is probably acceptable to have the one-and-only projection parse use the optimized MatchExpression, even if that means temporarily changing the query shape for such queries.&#160;&lt;/li&gt;
&lt;/ol&gt;
</description>
                <environment></environment>
        <key id="2344414">SERVER-83474</key>
            <summary>Can we avoid double-parsing a positional projection? Or alternatively can we afford to always double-parse the projection?</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-integration">Backlog - Query Integration</assignee>
                                    <reporter username="charlie.swanson@mongodb.com">Charlie Swanson</reporter>
                        <labels>
                            <label>qi-query-stats</label>
                            <label>query-product-scope-2</label>
                            <label>query-product-urgency-3</label>
                            <label>query-product-value-2</label>
                    </labels>
                <created>Thu, 18 May 2023 14:27:35 +0000</created>
                <updated>Thu, 4 Jan 2024 20:36:49 +0000</updated>
                                                                                                <votes>0</votes>
                                    <watches>3</watches>
                                                                                                                <comments>
                            <comment id="5893493" author="charlie.swanson" created="Mon, 20 Nov 2023 20:22:04 +0000"  >&lt;p&gt;Flagging this for scheduling to do this outside of PM-2885 or not at all. &lt;/p&gt;</comment>
                    </comments>
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                            <issuelinktype id="10012">
                    <name>Related</name>
                                                                <inwardlinks description="is related to">
                                        <issuelink>
            <issuekey id="2313447">SERVER-76042</issuekey>
        </issuelink>
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