Why is a query with indexes not a covered query? - mongodb

Question

You would like to perform a covered query on the example collection. You have the following indexes:
{ name : 1, dob : 1 }
{ _id : 1 }
{ hair : 1, name : 1 }
Why is the below query not a covered query?
db.example.find( { name : { $in : [ "Bart", "Homer" ] } }, {_id : 0, hair : 1, name : 1} )
While this one is:
db.example.find( { name : { $in : [ "Bart", "Homer" ] } }, {_id : 0, dob : 1, name : 1} )

Answer 1

According to documentation on index prefixes, the query
db.example.find( { name : { $in : [ "Bart", "Homer" ] } } );
will be covered by
db.example.createIndex({ "name": 1, "dob": 1 });
but not by
db.example.createIndex({ "hair": 1, "name": 1 });
since { "name": 1 } is not the preifx of { "hair": 1, "name": 1 }.
Sample
{ "_id": 1, "name": "Bart", "hair": "triangles", "dob": "1985-01-01" }
{ "_id": 2, "name": "Homer", "hair": "two", "dob": "1960-01-01" }
Query 1
> db.example.find(
> { name: { $in: [ "Bart", "Homer" ] } },
> { _id: 0, hair: 1, name: 1 }
> ).explain("executionStats");
...
"executionStats": {
"totalKeysExamined": 2,
"totalDocsExamined": 2,
"executionStages": {
"stage": "PROJECTION",
"inputStage": {
"stage": "FETCH",
"inputStage": {
"stage": "IXSCAN",
"indexName": "name_1_dob_1",
...
As you see, name_1_dob_1 index was used (since { "name": 1 } is the prefix of { "name": 1, "dob": 1 }, 2 documents were examined in the index ("totalKeysExamined": 2), and then 2 documents were examined in the collection ("totalDocsExamined": 2), since name_1_dob_1 index does not have information about hair that is required to return.
Query 2
> db.example.find(
> { name: { $in: [ "Bart", "Homer" ] } },
> { _id: 0, dob: 1, name: 1 }
> ).explain("executionStats");
...
"executionStats": {
"totalKeysExamined": 2,
"totalDocsExamined": 0,
"executionStages": {
"stage": "PROJECTION",
"inputStage": {
"stage": "IXSCAN",
"indexName": "name_1_dob_1",
...
As for the Query 1, index name_1_dob_1 was used and 2 documents were examined in the index ("totalKeysExamined": 2), but there were no call to the collection ("totalDocsExamined": 0), since index name_1_dob_1 contains both dob and name in it and there are no need to fetch something more from the collection.