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        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[DRIVERS-757] Ability to specify union</title>
                <link>https://jira.mongodb.org/browse/DRIVERS-757</link>
                <project id="10980" key="DRIVERS">Drivers</project>
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&lt;h4&gt;&lt;a name=&quot;Summary&quot;&gt;&lt;/a&gt;Summary&lt;/h4&gt;
&lt;p&gt;We will implement a new Agg stage, $union, that allows to merge results of n pipelines preserving duplicates. In order to enable merging data from multiple collections, we will also introduce an explicit stage to reference a collection, $collection.&lt;/p&gt;

&lt;h4&gt;&lt;a name=&quot;Motivation&quot;&gt;&lt;/a&gt;Motivation&lt;/h4&gt;
&lt;p&gt;Union is a fundamental operation in relational algebra. We have several specific scenarios:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;BIC connector for completeness with SQL.&lt;/li&gt;
	&lt;li&gt;TimeSeries scenario to combine data stored in per-period collections into one logical collection.&lt;/li&gt;
	&lt;li&gt;Combining collections in Data Lake, e.g. archival and recent data, data from different regions.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;For analytical scenarios, customers expect a complete set of fundamental operations. For example for Tableau, union and unpivot were top requested features after joins. In the future, we will be improving $lookup but delivering general and performant joins is a hard task. At the same  time, union-like logic is already supported for operations that require merging results across shards in the backend.&lt;/p&gt;


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        <key id="974280">DRIVERS-757</key>
            <summary>Ability to specify union</summary>
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                                            <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="-1">Unassigned</assignee>
                                    <reporter username="backlog-server-pm">Backlog - Core Eng Program Management Team</reporter>
                        <labels>
                    </labels>
                <created>Mon, 21 Oct 2019 14:43:17 +0000</created>
                <updated>Sat, 28 Oct 2023 10:45:10 +0000</updated>
                            <resolved>Thu, 30 Apr 2020 15:27:46 +0000</resolved>
                                                                            <votes>0</votes>
                                    <watches>3</watches>
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            <issuekey id="974550">JAVA-3472</issuekey>
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