$elemMatch forces mongo to fetch documents instead of using COUNT_SCAN

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    • Type: Bug
    • Resolution: Works as Designed
    • Priority: Major - P3
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    • Affects Version/s: 3.4.15
    • Component/s: Querying
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    • ALL
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      When perform "count" with "$elemmatch" in the query it never uses COUNT_SCAN and has to FETCH document before counting. Here is an example:

      //create a simple document that has an array of embedded doc, like this:
var doc = {
    "annotationList": [
        {
            "meta": {
                "league": "league_value1",
				"class": "class_value1"
            }
        },
        {
            "meta": {
                "league": "league_value1",
				"class": "class_value1"
            }
        }
    ]
}

//save doc to the collection
db.testCollection.save(doc);


//build compound index using 2 multikey field
db.testCollection.ensureIndex({"annotationList.meta.league": 1, "annotationList.meta.league": 1});

      Now if we perform the following:

       

      db.testCollection.explain("executionStats").count({
	
	"annotationList": {
		"$elemMatch": {
			"meta.league": "league_value1"
		}
	}
	
});

      We can see that it does the following: XSCAN -> FETCH -> COUNT

      However if we do the following:

      cursor = db.testCollection.explain("executionStats").count({
	
	"annotationList.meta.league": "league_value1"

});

       

      We can see that now there is not need to FETCH documents and it simply performs COUNT_SCAN

       

       

            Assignee:
            Asya Kamsky
            Reporter:
            Alexey [X]
            Votes:
            1 Vote for this issue
            Watchers:
            11 Start watching this issue

              Created:
              Updated:
              Resolved: