MongoDB - can not get a covered query - mongodb

Question

So I have an empty database 'tests' and a collection named 'test'.
First I ensured that my index was set correctly.
db.test.ensureIndex({t:1})
db.test.getIndices()
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "tests.test"
},
{
"v" : 1,
"key" : {
"t" : 1
},
"name" : "t_1",
"ns" : "tests.test"
}
]
After that I inserted some test records.
db.test.insert({t:1234})
db.test.insert({t:5678})
When I query the DB with following command and let Mongo explain the results I get the following output:
db.test.find({t:1234},{_id:0}).explain()
{
"cursor" : "BtreeCursor t_1",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 1,
"nscannedAllPlans" : 1,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"t" : [
[
1234,
1234
]
]
},
"server" : "XXXXXX:27017",
"filterSet" : false
}
Can anyone please explain to me why indexOnly is false?
Thanks in advance.

Answer 1

To be a covered index query you need to only retrieve those fields that are in the index:
> db.test.find({ t: 1234 },{ _id: 0, t: 1}).explain()
{
"cursor" : "BtreeCursor t_1",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 0,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 0,
"nscannedAllPlans" : 1,
"scanAndOrder" : false,
"indexOnly" : true,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"t" : [
[
1234,
1234
]
]
},
"server" : "ubuntu:27017",
"filterSet" : false
}
Essentially this means that only the index is used in order to retrieve the data, without the need to go back to the actual document and retrieve further information. This can be as many fields as you need ( within reason ), but they do need to be included within the index and the only fields that are returned.

Answer 2

Hmm the reason has not been clearly explained (confusing me actually) so here is my effort.
Essentially in order for MongoDB to know that said index covers the query it has to know what fields you want.
If you just say you don't want _id how can it know that * - _id = t without looking?
Here * represents all fields, like it does in SQL.
Answer is it cannot. That is why you need to provide the full field/select/projection/whatever word they use for it definition so that MongoDB can know that your return fits the index.