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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-78353] [CQF] Investigate reference tracker stack-overflow</title>
                <link>https://jira.mongodb.org/browse/SERVER-78353</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;The maximum number of aggregation stages allowed is 1000 for non-debug builds. For the following pipeline (and other long pipelines) the classic engine has no issues, while Bonsai fails with a stack overflow in the &lt;a href=&quot;https://github.com/10gen/mongo/blob/df2e601d78a38db6cb4173bb889d565a01e7b6a1/src/mongo/db/query/optimizer/opt_phase_manager.cpp#L295&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;first call to the reference tracker&lt;/a&gt;.&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: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;db.c.drop();&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: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;const&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; stage = {$addFields: {a: {$add: [&lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;$a&quot;&lt;/span&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;, &lt;/span&gt;&lt;span style=&quot;color: blue; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;&quot;$b&quot;&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;
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				&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;// Make a pipeline with 990 stages (all the same stage)&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: #006699; font-weight: bold; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;const&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; pipeline = [...Array(&lt;/span&gt;&lt;span style=&quot;color: #009900; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;990&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;).keys()].map(_ =&amp;gt; stage);&lt;/span&gt;&lt;/pre&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;   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.c.aggregate(pipeline).toArray();&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;// Stack overflow&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;
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&lt;p/&gt;

&lt;p&gt;We should investigate why the stack frames are so large. Are we allocating unnecessarily at all?&lt;/p&gt;

&lt;p&gt;The overall goal is to determine if the transport infrastructure needs to fundamentally change to be able to handle deep trees, or is there a less extreme change we can make?&lt;/p&gt;</description>
                <environment></environment>
        <key id="2375044">SERVER-78353</key>
            <summary>[CQF] Investigate reference tracker stack-overflow</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="9">Done</resolution>
                                        <assignee username="david.percy@mongodb.com">David Percy</assignee>
                                    <reporter username="matt.boros@mongodb.com">Matt Boros</reporter>
                        <labels>
                    </labels>
                <created>Thu, 22 Jun 2023 19:06:21 +0000</created>
                <updated>Tue, 29 Aug 2023 18:35:01 +0000</updated>
                            <resolved>Tue, 29 Aug 2023 18:35:01 +0000</resolved>
                                                                                        <votes>0</votes>
                                    <watches>7</watches>
                                                                                                                <comments>
                            <comment id="5666749" author="david.percy" created="Tue, 29 Aug 2023 18:35:01 +0000"  >&lt;p&gt;After some discussion we&apos;ve decided to:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;Try changing the implementation of algebra::transport to handle trees of any depth: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-78354&quot; title=&quot;[CQF] Implement transport infrastructure using iteration rather than recursion&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-78354&quot;&gt;&lt;del&gt;SERVER-78354&lt;/del&gt;&lt;/a&gt;.  As long as we can preserve the interface exactly.&lt;/li&gt;
	&lt;li&gt;If that doesn&apos;t work out or is not worth doing right now, change the reftracker transport to use unique_ptr.  It may be wasteful to allocate those temp objects but it would definitely fix the stack overflow, and we can come back to it if necessary.&lt;/li&gt;
&lt;/ul&gt;
</comment>
                            <comment id="5532422" author="JIRAUSER1253388" created="Wed, 28 Jun 2023 20:58:00 +0000"  >&lt;p&gt;I&apos;ve found that this is also an issue for M2 queries. Repeating {$project: {a: 1}} one thousand times also causes stack overflow in the reference tracker.&lt;/p&gt;</comment>
                            <comment id="5519299" author="JIRAUSER1253388" created="Thu, 22 Jun 2023 19:09:23 +0000"  >&lt;p&gt;Note that this failure occurs in the &lt;em&gt;first&lt;/em&gt; call to the reference tracker, before any rewrites or real work is done by the optimizer. We may fix this issue with the reference tracker and still have overflow issues with Bonsai for this query.&lt;/p&gt;</comment>
                    </comments>
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            <issuekey id="1961583">SERVER-62509</issuekey>
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            <issuekey id="2428514">SERVER-80483</issuekey>
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