[SERVER-35223] $elemMatch forces mongo to fetch documents instead of using COUNT_SCAN - MongoDB Jira

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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

is related to

SERVER-37290 Avoid FETCH stage with $elemmatch if using covered index and multiKeyPaths is correct

related to

SERVER-27494 Avoid unwind on multi-key index field for covered index aggregation

SERVER-41594 Covered query don't use index for range $gt+$lt. Only for one side and FETCH filter for another