How does MongoDB choose the index to use?

[Bogdan Ciobanu]

Hi,
I am learning to use MongoDB on a collection containing documents with the following structure:

{
    "_id" : ObjectId("53a18d636171cfe49d7647b6"),
    "datetime" : "2011-01-01 00:00:00",
    "season" : 1,
    "holiday" : 0,
    "workingday" : 0,
    "weather" : 1,
    "temp" : 9.84,
    "atemp" : 14.395,
    "humidity" : 81,
    "windspeed" : 0,
    "casual" : 3,
    "registered" : 13,
    "count" : 16
}

and I have defined 3 indexes:

db.collection.ensureIndex({temp:1})
db.collection.ensureIndex({count:1})
db.collection.ensureIndex({temp:1, count:1})

Based on the fact that the documentation specifies that "MongoDB automatically uses an index that covers a query when possible" [1], I was expecting the following query to be indexOnly:

db.bikes.find({temp: {$gt:23}}, {temp:1, count:1, _id: 0})

Unfortunately, it uses the ({temp: 1}) index and it obviously doesn't cover the query.

Is there any logic under the index used and in what way is Mongo using an index that covers a query?

[1] - http://docs.mongodb.org/manual/tutorial/create-indexes-to-support-queries/#create-indexes-that-support-covered-queries

[Asya Kamsky]

Are you using 2.6.1?   If so I would try to use 2.6.2 and see if the behavior changes.

If not, then run the same query with explain(true) and post all the plans here - I'm assuming this query actually matches more than one document, right?

Asya

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