Overview:

The 'nscannedObjects' field in the explain output for a query describes the number of documents that would be read from the data store to perform a query. (This count does not include cases where an index key is read but the document that index key comes from is not read.) The 'nscannedObjects' field is reported on a per query basis and also on a per query plan basis (in the allPlans section of the explain output).

Reporting nscannedObjects is not implemented by checking directly if the data layer has read a document, but by determining if the document would have been read at different stages of query handling on a case by case basis. Per this ticket, nscannedObjects is now reported as follows:

• for a query, the nscannedObjects count includes cases where a document is read to perform query matching, a document is read to determine if it belongs to a valid chunk, or a document would be read to return a result (including all documents used to perform an in memory sort, but not covered index projections that aren't sorted in memory).

• for a query plan, the nscannedObjects count includes cases where a document is read to perform query matching or a document must be read to return a result (as determined here <https://github.com/mongodb/mongo/blob/6e555988bff53d807635ef59796c45c824b770ba/src/mongo/db/queryoptimizercursorimpl.cpp#L271-L290>).

-----------------------------------

Expected nscannedObjects to be 0 for a covered index query but it is 1.

for (var i = 1; i < 11; i++) {
db.cindextest.insert(

{foo:i,bar:i*10}

)
}
db.cindextest.ensureIndex(

{foo:1}

)

> db.cindextest.find(

{foo:1}

,

{foo:1,_id:0}

).explain()
{
"cursor" : "BtreeCursor foo_1",
"nscanned" : 1,
"nscannedObjects" : 1, <------------- expected this to be 0
"n" : 1,
"millis" : 0,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : true,
"indexBounds" :

{ "foo" : [ [ 1, 1 ] ] }

}