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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-4428] Queries executed via MongoS will always project the sort key</title>
                <link>https://jira.mongodb.org/browse/DOCS-4428</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;p&gt;This is a &quot;gotcha&quot; when using index filters and evaluating query performance.&lt;/p&gt;

&lt;p&gt;When you issue a query via a MongoS that includes both a Projection and a Sort field, the sort field will always be included within the projection set to the shards. This is needed for sorting on the result data from shards by desired field on the MongoS.&lt;/p&gt;

&lt;p&gt;This means when you are adding index filters using projections, you should always expect to add the sort field to the projection in sharded environments.&lt;/p&gt;</description>
                <environment></environment>
        <key id="171106">DOCS-4428</key>
            <summary>Queries executed via MongoS will always project the sort key</summary>
                <type id="3" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14718&amp;avatarType=issuetype">Task</type>
                                            <priority id="4" iconUrl="https://jira.mongodb.org/images/icons/priorities/minor.svg">Minor - P4</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="andrew.aldridge@mongodb.com">Andrew Aldridge</assignee>
                                    <reporter username="david.hows">David Hows</reporter>
                        <labels>
                            <label>query</label>
                    </labels>
                <created>Tue, 25 Nov 2014 02:21:02 +0000</created>
                <updated>Mon, 30 Oct 2023 22:31:22 +0000</updated>
                            <resolved>Tue, 21 May 2019 18:51:14 +0000</resolved>
                                                    <fixVersion>Server_Docs_20231030</fixVersion>
                                    <component>manual</component>
                        <due></due>
                            <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="1194995" author="david.storch" created="Mon, 7 Mar 2016 15:17:27 +0000"  >&lt;p&gt;This behavior is no longer present in mongos as of 3.2. As part of find/getMore commands project (SERVER-15176), we rewrote the find path on mongos. This rewrite changed how mongos obtains sort keys for performing its sorted merge.&lt;/p&gt;

&lt;p&gt;Consider the following example query:&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;   margin-bottom: 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.coll.find({}, {_id: 0, b: 1}).sort({a: 1});&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;This query both projects out &lt;tt&gt;a&lt;/tt&gt; and sorts by it. Past versions of mongos would amend the projection to {_id: 0, b: 1, a: 1} so that the sort keys are returned to mongos in order to perform the sorted merge. 3.2.x and newer versions of mongos instead use something called sortKey $meta projection. In these newer versions, mongos will amend the projection to something like {_id: 0, b: 1, $sortKey: {$meta: &apos;sortKey&apos;}}. This will cause the shards to exclude the &lt;tt&gt;a&lt;/tt&gt; field but add a new field called &quot;$sortKey&quot; which contains the internally-generated sort key for each document. The sorted merge proceeds on mongos using this &quot;$sortKey&quot; information.&lt;/p&gt;</comment>
                            <comment id="903866" author="charlie.page@10gen.com" created="Sun, 3 May 2015 05:10:14 +0000"  >&lt;p&gt;Not this simple.  Query filters are &lt;b&gt;not&lt;/b&gt; type insensitive, and they should be.  Therefore the query filters have to match the &lt;b&gt;type&lt;/b&gt; that mognoS uses (which probably isn&apos;t what users are using...).  I linked &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-13008&quot; title=&quot;Encoding of projection for query plan cache should be type-insensitive&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-13008&quot;&gt;&lt;del&gt;SERVER-13008&lt;/del&gt;&lt;/a&gt;.  I don&apos;t recall the type mongoS uses, but { a : 1 } doesn&apos;t match.&lt;/p&gt;</comment>
                    </comments>
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            <issuekey id="170664">SERVER-16270</issuekey>
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            <issuekey id="744944">DOCS-12652</issuekey>
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            <issuekey id="117009">SERVER-13008</issuekey>
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