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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>[DOCS-14310] Call out non-guaranteed order of $unionWith</title>
                <link>https://jira.mongodb.org/browse/DOCS-14310</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;h2&gt;&lt;a name=&quot;Description&quot;&gt;&lt;/a&gt;Description&lt;/h2&gt;
&lt;p&gt;Currently, $unionWith outputs documents in a predictable order.  But it&apos;s not guaranteed: users should assume that $unionWith can output documents in any order.&lt;/p&gt;

&lt;p&gt;The page for &lt;a href=&quot;https://docs.mongodb.com/manual/reference/operator/aggregation/unionWith/index.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;$unionWith&lt;/a&gt; doesn&apos;t mention ordering one way or the other.  It would be good to explicitly warn people that the order is not guaranteed.  (We&apos;ve done this for other operators, like &lt;a href=&quot;https://docs.mongodb.com/manual/reference/operator/aggregation/addToSet/index.html&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;$addToSet&lt;/a&gt;.)&lt;/p&gt;

&lt;p&gt;The &lt;a href=&quot;https://docs.google.com/document/d/10heZ18gZQw3x4MFPaTcY1Pwb0XCW7aSsEX5-cSe-nvs/edit#heading=h.qo2k1tdfy1ja&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;Versioned API&lt;/a&gt; Scope talks about why we don&apos;t want to promise the output order will stay predictable: it leaves the door open for us to optimize it better in the future.&lt;/p&gt;

&lt;h2&gt;&lt;a name=&quot;Scopeofchanges&quot;&gt;&lt;/a&gt;Scope of changes&lt;/h2&gt;

&lt;h2&gt;&lt;a name=&quot;ImpacttoOtherDocs&quot;&gt;&lt;/a&gt;Impact to Other Docs&lt;/h2&gt;

&lt;h2&gt;&lt;a name=&quot;MVP%28WorkandDate%29&quot;&gt;&lt;/a&gt;MVP (Work and Date)&lt;/h2&gt;

&lt;h2&gt;&lt;a name=&quot;Resources%28ScopeorDesignDocs%2CInvision%2Cetc.%29&quot;&gt;&lt;/a&gt;Resources (Scope or Design Docs, Invision, etc.)&lt;/h2&gt;
</description>
                <environment></environment>
        <key id="1657945">DOCS-14310</key>
            <summary>Call out non-guaranteed order of $unionWith</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="sarah.olson@mongodb.com">Sarah Olson</assignee>
                                    <reporter username="david.percy@mongodb.com">David Percy</reporter>
                        <labels>
                    </labels>
                <created>Wed, 24 Mar 2021 16:31:53 +0000</created>
                <updated>Mon, 13 Nov 2023 17:56:41 +0000</updated>
                            <resolved>Tue, 8 Feb 2022 20:26:40 +0000</resolved>
                                                    <fixVersion>4.4.0</fixVersion>
                    <fixVersion>5.0.0</fixVersion>
                    <fixVersion>5.2.0</fixVersion>
                    <fixVersion>Server_Docs_20231030</fixVersion>
                    <fixVersion>Server_Docs_20231106</fixVersion>
                    <fixVersion>Server_Docs_20231105</fixVersion>
                    <fixVersion>Server_Docs_20231113</fixVersion>
                                    <component>manual</component>
                    <component>Server</component>
                        <due></due>
                            <votes>0</votes>
                                    <watches>1</watches>
                                                                                                                <comments>
                            <comment id="4343435" author="JIRAUSER1264463" created="Tue, 8 Feb 2022 20:26:40 +0000"  >&lt;p&gt;This has been reviewed, backported, and merged. Closing as fixed. Please feel free to give a shout with any additional comments.&lt;/p&gt;</comment>
                            <comment id="4343429" author="xgen-internal-githook" created="Tue, 8 Feb 2022 20:24:39 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Sarah Olson&apos;, &apos;email&apos;: &apos;98367156+sarah-olson-mongodb@users.noreply.github.com&apos;, &apos;username&apos;: &apos;sarah-olson-mongodb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Add output order disclaimer. (#495) (#563)&lt;/p&gt;

&lt;ul&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Add output order disclaimer.&lt;/li&gt;
&lt;/ul&gt;


&lt;ul&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Update to include PR review edits&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Co-authored-by: Sarah Olson &amp;lt;sarah.olson@Sarahs-MacBook-Pro.local&amp;gt;&lt;/p&gt;

&lt;p&gt;Co-authored-by: Sarah Olson &amp;lt;sarah.olson@Sarahs-MacBook-Pro.local&amp;gt;&lt;br/&gt;
Branch: v4.4&lt;br/&gt;
&lt;a href=&quot;https://github.com/10gen/docs-mongodb-internal/commit/91b05200fd62e2ca79731a3e39059c351ef4a08c&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/10gen/docs-mongodb-internal/commit/91b05200fd62e2ca79731a3e39059c351ef4a08c&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="4343427" author="xgen-internal-githook" created="Tue, 8 Feb 2022 20:24:10 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Sarah Olson&apos;, &apos;email&apos;: &apos;98367156+sarah-olson-mongodb@users.noreply.github.com&apos;, &apos;username&apos;: &apos;sarah-olson-mongodb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Add output order disclaimer. (#495) (#562)&lt;/p&gt;

&lt;ul&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Add output order disclaimer.&lt;/li&gt;
&lt;/ul&gt;


&lt;ul&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Update to include PR review edits&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Co-authored-by: Sarah Olson &amp;lt;sarah.olson@Sarahs-MacBook-Pro.local&amp;gt;&lt;/p&gt;

&lt;p&gt;Co-authored-by: Sarah Olson &amp;lt;sarah.olson@Sarahs-MacBook-Pro.local&amp;gt;&lt;br/&gt;
Branch: v5.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/10gen/docs-mongodb-internal/commit/75f2054a5fcc422bd0d2c352d8dc58c2e147b91a&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/10gen/docs-mongodb-internal/commit/75f2054a5fcc422bd0d2c352d8dc58c2e147b91a&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="4343425" author="xgen-internal-githook" created="Tue, 8 Feb 2022 20:23:51 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Sarah Olson&apos;, &apos;email&apos;: &apos;98367156+sarah-olson-mongodb@users.noreply.github.com&apos;, &apos;username&apos;: &apos;sarah-olson-mongodb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Add output order disclaimer. (#495) (#561)&lt;/p&gt;

&lt;ul&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Add output order disclaimer.&lt;/li&gt;
&lt;/ul&gt;


&lt;ul&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Update to include PR review edits&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Co-authored-by: Sarah Olson &amp;lt;sarah.olson@Sarahs-MacBook-Pro.local&amp;gt;&lt;/p&gt;

&lt;p&gt;Co-authored-by: Sarah Olson &amp;lt;sarah.olson@Sarahs-MacBook-Pro.local&amp;gt;&lt;br/&gt;
Branch: v5.2&lt;br/&gt;
&lt;a href=&quot;https://github.com/10gen/docs-mongodb-internal/commit/4ba75133bcec88e83135045e62e660ee148eb008&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/10gen/docs-mongodb-internal/commit/4ba75133bcec88e83135045e62e660ee148eb008&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="4333858" author="xgen-internal-githook" created="Thu, 3 Feb 2022 17:43:12 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Sarah Olson&apos;, &apos;email&apos;: &apos;98367156+sarah-olson-mongodb@users.noreply.github.com&apos;, &apos;username&apos;: &apos;sarah-olson-mongodb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Add output order disclaimer. (#495)&lt;/p&gt;

&lt;ul&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Add output order disclaimer.&lt;/li&gt;
&lt;/ul&gt;


&lt;ul&gt;
	&lt;li&gt;&lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-14310&quot; title=&quot;Call out non-guaranteed order of $unionWith&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-14310&quot;&gt;&lt;del&gt;DOCS-14310&lt;/del&gt;&lt;/a&gt;: Update to include PR review edits&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Co-authored-by: Sarah Olson &amp;lt;sarah.olson@Sarahs-MacBook-Pro.local&amp;gt;&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/10gen/docs-mongodb-internal/commit/10cc64e606a5f35188881e801c72301849fe003f&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/10gen/docs-mongodb-internal/commit/10cc64e606a5f35188881e801c72301849fe003f&lt;/a&gt;&lt;/p&gt;</comment>
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