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        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-4408] _id index should be used for find queries against nested fields of the _id object</title>
                <link>https://jira.mongodb.org/browse/SERVER-4408</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p/&gt;
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&lt;p/&gt;


&lt;p&gt;When using a BSONObject as the _id for the record, querying against only a part of the object should still use the _id btree index. I appreciate that because the existence and order of fields in the nested document is not known, this will not be a range scan (not even for an _id field of the nested document), but a &quot;fast-full-index-scan&quot; type of access path should be possible and faster than a table scan.&lt;/p&gt;




</description>
                <environment></environment>
        <key id="25859">SERVER-4408</key>
            <summary>_id index should be used for find queries against nested fields of the _id object</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</type>
                                            <priority id="4" iconUrl="https://jira.mongodb.org/images/icons/priorities/minor.svg">Minor - P4</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-optimization">Backlog - Query Optimization</assignee>
                                    <reporter username="thiloplanz">Thilo Planz</reporter>
                        <labels>
                    </labels>
                <created>Fri, 2 Dec 2011 01:53:36 +0000</created>
                <updated>Tue, 6 Dec 2022 05:38:59 +0000</updated>
                                            <version>2.0.1</version>
                                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>6</watches>
                                                                                                                <comments>
                            <comment id="70315" author="thiloplanz" created="Fri, 2 Dec 2011 02:01:40 +0000"  >&lt;p&gt;Come to think of it, this reasoning does not only apply for the _id index, but for any kind of index built on nested documents. &lt;/p&gt;

&lt;p&gt;And the workaround would be to build an extra index on the nested field to be queried:&lt;/p&gt;

&lt;p&gt;&amp;gt; db.textarea.vermongo.ensureIndex(&lt;/p&gt;
{&apos;_id._id&apos;: 1}
&lt;p&gt;);&lt;br/&gt;
&amp;gt; db.textarea.vermongo.find( &lt;/p&gt;
{&apos;_id._id&apos;: &apos;y&apos;}
&lt;p&gt;).explain();&lt;br/&gt;
{&lt;br/&gt;
	&quot;cursor&quot; : &quot;BtreeCursor _id._id_1&quot;,&lt;/p&gt;

&lt;p&gt;However, I was putting these nested documents into the _id field mainly for the purpose of not requiring an additional index in addition to the mandatory _id index.&lt;/p&gt;

&lt;p&gt;(And the priority for all this is low, because I can achieve what I want to do by doing an explicit range query on the _id field, as I am querying an the first field of the nested document, so binary sort order in the _id index serves fine. However, if I wanted to query on the second field it would be nice to have a index fast-full scan).&lt;/p&gt;</comment>
                    </comments>
                <issuelinks>
                            <issuelinktype id="10010">
                    <name>Duplicate</name>
                                                                <inwardlinks description="is duplicated by">
                                        <issuelink>
            <issuekey id="26853">SERVER-4518</issuekey>
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