Mongodb covered the execution query Stats – Stack Exchange database administrators

Hello, I would like some help to understand the following execution statistics.

For this query ({"_id": ObjectId ("5c87890b3adcde7da566f0d7")}, {_ id: 1}). explain ("ecutionStats")
                                "ecutionSuccess": true,
"nReturn": 1,
"ecutionTimeMillis": 0,
"totalKeysExamined": 1,
"totalDocsExamined": 1,

Specifically, I am not sure why totalDocsExamined is 1 because I would have imagined that if there is no index in ObjectId, and it is only returned, is not it necessary to examine any document?

Please help