Why a query is not covered when using a range operator on documents?

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                          I have created the following 1000 documents:

    for(i=0;i<1000;i++){db.doc.insert({"doc":{"k1":"v_"+i,"k2":i},"scal":i})}


So a document looks like this:

    var d1 = db.doc.findOne()
    {
    	"_id" : ObjectId("5ce25f8920b0be2428648e38"),
    	"doc" : {
    		"k1" : "v_0",
    		"k2" : 0
    	},
    	"scal" : 0
    }

I've created the following two indexes:

    db.doc.createIndex({"doc":1})
    db.doc.createIndex({"scal":1})


I wonder why the following query is not covered:


    db.doc.explain(true).find({doc:{$gt:d1.doc}},{_id:0,doc:1})
    {
    	...
    	"executionStats" : {
    		"executionSuccess" : true,
    		"nReturned" : 999,
    		"executionTimeMillis" : 1,
    		"totalKeysExamined" : 999,
    		"totalDocsExamined" : 999,
    		...
    }

However, the same query using a scalar instead of a document is covered:

    db.doc.explain(true).find({scal:{$gt:d1.scal}},{_id:0,scal:1})
    {
    	...
    	"executionStats" : {
    		"executionSuccess" : true,
    		"nReturned" : 999,
    		"executionTimeMillis" : 0,
    		"totalKeysExamined" : 999,
    		"totalDocsExamined" : 0,
    		...
    }

I only get the query covered on documents, when I'm **not** using a range operator:

    db.doc.explain(true).find({doc:d1.doc},{_id:0,doc:1})
    {
    	...
    	"executionStats" : {
    		"executionSuccess" : true,
    		"nReturned" : 1,
    		"executionTimeMillis" : 0,
    		"totalKeysExamined" : 1,
    		"totalDocsExamined" : 0,
    		...
    }

Even $in would result in a covered query:

    var dis = db.doc.distinct("doc")
    db.doc.explain(true).find({doc:{$in:dis}},{_id:0,doc:1})
    {
    ...
    "executionStats" : {
    		"executionSuccess" : true,
    		"nReturned" : 1000,
    		"executionTimeMillis" : 8,
    		"totalKeysExamined" : 1000,
    		"totalDocsExamined" : 0,
    		...
    }

However range operators such as $gt seem not to be able to result in a covered query when applied to documents. Is this an expected behaviour which is documented anywhere or is it a bug?

I'm using a replSet running with mongodb v3.6 Linux 64 Bit.
                        

Asked by Kay (211 rep)

May 20, 2019, 09:12 AM
Last activity: May 20, 2019, 06:10 PM

Why a query is not covered when using a range operator on documents?

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