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  1. Core Server
  2. SERVER-42568

Covered query for $replaceRoot involving missing fields returns different results than collection scan

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    • Type: Icon: Bug Bug
    • Resolution: Duplicate
    • Priority: Icon: Major - P3 Major - P3
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    • Affects Version/s: None
    • Component/s: None
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      python buildscripts/resmoke.py --storageEngine wiredTiger --suites aggregation repro_match_replace_root.js
      repro_match_replace_root.js
      assert.commandWorked(db.my_coll.insert({}));

const res1 = db.my_coll
	.aggregate([
		{
			$match: {
				a: { 
					$not: { 
						$gte: 5
					}
				}
			}
		},
		{
			$replaceRoot: {
				newRoot:{
					v: '$a'
				}
			}
		},
]).toArray();

assert.commandWorked(db.my_coll.createIndex({a:1}));

const res2 = db.my_coll
	.aggregate([
		{
			$match: {
				a: { 
					$not: { 
						$gte: 5
					}
				}
			}
		},
		{
			$replaceRoot: {
				newRoot:{
					v: '$a'
				}
			}
		},
]).toArray();

assert.eq(res1, res2);
      Show
      python buildscripts/resmoke.py --storageEngine wiredTiger --suites aggregation repro_match_replace_root.js repro_match_replace_root.js assert.commandWorked(db.my_coll.insert({})); const res1 = db.my_coll .aggregate([ { $match: { a: { $not: { $gte: 5 } } } }, { $replaceRoot: { newRoot:{ v: '$a' } } }, ]).toArray(); assert.commandWorked(db.my_coll.createIndex({a:1})); const res2 = db.my_coll .aggregate([ { $match: { a: { $not: { $gte: 5 } } } }, { $replaceRoot: { newRoot:{ v: '$a' } } }, ]).toArray(); assert.eq(res1, res2);
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      When no index is present and $match is called before a $replaceRoot that uses the same index, it will do a collection scan and $replaceRoot will return an empty document. However, when the index is present but the field doesn't exist in the document, the query system ends up performing a covered query. The index has a null entry for the document because the field doesn't exist in the document. This causes $replaceRoot to return a non-empty document with the missing field associated with a literal null value.

      [js_test:repro_match_replace_root] 2019-07-31T18:09:25.364-0400 2019-07-31T18:09:25.363-0400 E  QUERY    [js] uncaught exception: Error: [[ { } ]] != [[ { "v" : null } ]] are not equal :
[js_test:repro_match_replace_root] 2019-07-31T18:09:25.364-0400 doassert@src/mongo/shell/assert.js:20:14
[js_test:repro_match_replace_root] 2019-07-31T18:09:25.364-0400 assert.eq@src/mongo/shell/assert.js:180:9
[js_test:repro_match_replace_root] 2019-07-31T18:09:25.364-0400 @repro_match_replace_root.js:45:1
[js_test:repro_match_replace_root] 2019-07-31T18:09:25.364-0400 2019-07-31T18:09:25.363-0400 F  -        [main] failed to load: repro_match_replace_root.js
[js_test:repro_match_replace_root] 2019-07-31T18:09:25.364-0400 2019-07-31T18:09:25.363-0400 E  -        [main] exiting with code -3

            Assignee:
            backlog-server-query Backlog - Query Team (Inactive)
            Reporter:
            sandy.gould@mongodb.com Sandra Gould (Inactive)
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            Watchers:
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              Created:
              Updated:
              Resolved: