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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-19593] Allow collscans on $snapshot queries when not using MMAP1</title>
                <link>https://jira.mongodb.org/browse/SERVER-19593</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;In MMAPv1 we need to scan the _id index to ensure we see documents once since records can move in the collection, allowing us to see duplicates or miss documents. In other storage engines this is not allowed, so we should be able to do table scans rather than index scans which can be orders of magnitude slower.&lt;/p&gt;

&lt;p&gt;This would result in a user-facing change since we would no longer be returning documents in _id order, but that is not part of the documented behavior of $snapshot.&lt;/p&gt;</description>
                <environment></environment>
        <key id="222643">SERVER-19593</key>
            <summary>Allow collscans on $snapshot queries when not using MMAP1</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="9">Done</resolution>
                                        <assignee username="qingyang.chen">Qingyang Chen</assignee>
                                    <reporter username="mathias@mongodb.com">Mathias Stearn</reporter>
                        <labels>
                    </labels>
                <created>Fri, 24 Jul 2015 18:46:19 +0000</created>
                <updated>Wed, 22 Mar 2017 15:26:26 +0000</updated>
                            <resolved>Thu, 6 Aug 2015 17:29:52 +0000</resolved>
                                    <version>3.0.0</version>
                                    <fixVersion>3.1.7</fixVersion>
                                    <component>Querying</component>
                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="1155352" author="xgen-internal-githook" created="Wed, 27 Jan 2016 16:05:58 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;ksuarz&apos;, u&apos;name&apos;: u&apos;Kyle Suarez&apos;, u&apos;email&apos;: u&apos;ksuarz@gmail.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-7000&quot; title=&quot;cursor.snapshot() page should no longer reference _id index&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-7000&quot;&gt;&lt;del&gt;DOCS-7000&lt;/del&gt;&lt;/a&gt; no _id index scan for cursor.snapshot()&lt;/p&gt;

&lt;p&gt;Due to &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-19593&quot; title=&quot;Allow collscans on $snapshot queries when not using MMAP1&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-19593&quot;&gt;&lt;del&gt;SERVER-19593&lt;/del&gt;&lt;/a&gt;, the cursor.snapshot() method no longer forces an _id index&lt;br/&gt;
scan and instead performs a collection scan.&lt;/p&gt;

&lt;p&gt;This removes the description of the implementation of cursor.snapshot(). It also&lt;br/&gt;
removes the extract isolate-cursor-snapshot-faq.&lt;/p&gt;

&lt;p&gt;Signed-off-by: kay &amp;lt;kay.kim@10gen.com&amp;gt;&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/ef3f09b9236684ddda0b9df50bfff3beb554af69&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/ef3f09b9236684ddda0b9df50bfff3beb554af69&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="998504" author="xgen-internal-githook" created="Thu, 6 Aug 2015 17:29:24 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;coollog&apos;, u&apos;name&apos;: u&apos;Qingyang Chen&apos;, u&apos;email&apos;: u&apos;qingyang.chen@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-19593&quot; title=&quot;Allow collscans on $snapshot queries when not using MMAP1&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-19593&quot;&gt;&lt;del&gt;SERVER-19593&lt;/del&gt;&lt;/a&gt; snapshot default to collscan except when using MMAPv1&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/2830b503d24bd33077a4161e60ad4727972d5508&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/2830b503d24bd33077a4161e60ad4727972d5508&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="991227" author="qingyang.chen" created="Tue, 28 Jul 2015 20:17:31 +0000"  >&lt;p&gt;Method:&lt;br/&gt;
1) Add QueryPlannerParams parameter, SNAPSHOT_USE_IXSCAN, have snapshot do as before (index on _id) if isMMAPV1() (get_executor.cpp:fillOutPlannerParams)&lt;br/&gt;
2) Change snapshot() to default to be hint($natural) (code in QueryPlanner::plan)&lt;/p&gt;</comment>
                    </comments>
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            <issuekey id="224270">SERVER-19781</issuekey>
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                                        <issuelink>
            <issuekey id="259371">DOCS-7000</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="259118">SERVER-22214</issuekey>
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            <issuekey id="222569">TOOLS-845</issuekey>
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